So, the most obvious cue is that Jake <3 Vriska is extremely foreshadowed, and part of that foreshadowing is Jake's reaction to seeing Aranea, who looks just like her descendent. Pretty much as soon as he sees her, he freaks out over how hot and exactly his type she is, while she is watching and listening and politely not saying anything. So, Jake's redcrush on Aranea = Big Indication of Red VrisJake, yeah?

Well, when Roxy sees Aranea, she has a similarly strong reaction.

ROXY: no no ROXY: no no no no ROXY: oh man i rly hate you ROXY: i cant even explain it i never hated somebody so much by just a few smarmy words outta their grody chew hole ROXY: eugh yechh ick ick blarff!

And Aranea does, in fact, take it as an expression of pitch attraction.

ARANEA: That was quite the 8lack solicit8tion though.

And, in fact, directly compares it to Jake's flustered freakout.

ARANEA: It does get to 8e a little 8othersome having people fling themselves at you all the time. It's always 8een my curse. ARANEA: Did you know just earlier today, your friend Jake could 8arely keep his hands off me? ;;;;)

So, yeah, like, if we're accepting that Jake losing his shit over Aranea is red foreshadowing of VrisJake, then Roxy's similar hatred freakout at Aranea puts black VrisRoxy on the same level.

But, like VrisJake, there's more than just that. Who is the other person that Roxy hates more than anybody else, to the point of bombing Jane in protest? The Batterwitch, of course... a Thief. And we know from several other pairings foreshadowed by crushes (VrisJake, DirkTav, EriRox, JaneEquius) that classes are often a huge indicator of a character's "type".

The Vriska side of thing is a bit murkier, because Vriska is kind of just desperate for attention, and tends to take both negative and positive attention indiscriminately. As a result, she's got a major problem with antagonizing nearly everybody, and people are overwhelmingly hostile to her in turn. As Karkat says, she does a lot of poser-hatred without actually hating people.

So when dealing with a character who has a pattern of acting hateful and antagonistic to people she doesn't really hate, and who also clearly loves and craves positive attention, and can therefore be seen trying to win it from people she respects, then what we're looking for vis-a-vis someone she actually dislikes will be avoidance: Vriska doesn't even WANT their attention. And there's one person who fits the bill.

AG: Now I have to contact you through Rose, thus exposing me to the risk of actually having to taaaaaaaalk to her........ AG: Your carelessness has put the Heroes of Light in a very awkward position, John. I hope you're satisfied. AG: Hahahaha, just kidding. She's o8viously a little too "preoccupied" at the moment to 8e sassing me.

Now, again, another common indicator for foreshadowed endgame ship is a character showing a milder form of interest in a family member - John and Dave almost have a pale thing, Dirk and Jane almost have a pale thing, John and Vriska have their red fling, Tavros can't seem to make up his mind whether he loves or hates Dave.

So, Rose is the only person Vriska doesn't actually want to talk to, implied to be because Vriska - the spotlight hog - feels displeasure at the idea of sharing with someone who has a similar role.

Well, Roxy isn't exactly a hero of Light, but Void is Light's matched opposite, and a Rogue is a passive Thief. As Calliope says to Roxy directly:

UU: for instance, a hero of life and a hero of doom have aspects as different as can be. UU: bUt if their classes are different enoUgh as well, that is, one active and the other passive, remarkably there is a chance they coUld end Up with very similar abilities!

Rogues, especially, are all about subversion. Roxy literally has the ability to steal the Void from stuff, which makes it exist/be more relevant. If Vriska's problem with Rose is that she has difficulty grappling with someone who shares her spotlight, a Rogue of Void is actually much closer than a Seer of Light. Plus, they share some major personality traits - recklessness, willingness to take a stand, stubborn refusal to listen to authority...

Plus, there's something interesting to note in [S] Past Karkat: Wake Up - Vriska, multiple times, tries to entreat people to interact with her and give her attention. Multiple times, people either decline, or show her outright hostility. And multiple times, even though she's obviously hurt, she's able to put on a brave face and persist.

Except for when Nepeta has a go at her.

VRISKA: How a8out my spidersona? It's adora8le! NEPETA: :33 < no its not! you only do terrible things as your spidersona too NEPETA: :33 < i changed my mind, why dont we just not do any roleplaying instead VRISKA: ::::(

Nepeta is, after all, a cat-themed Rogue.

And finally, something to look for in blackroms is some degree of irreconcileable opposites - using GamTer as the most canon example (a parallel relationship to RoseMary), Terezi is obsessed with law and justice, and her role on the team is explicitly "keeping everyone out of trouble". Gamzee, meanwhile, is described by Hussie as "inscrutable", "chaotic," "capricious", and a saboteur, constantly showing up at key points in the story to ruin everything.

Most of the classpects don't work out so neatly, but Vriska is literally a Thief of Light going up against a Rogue of Void. She causes problems on purpose for everyone else so she can take the credit, while Roxy quietly and unassumingly fixes her own shit in the background.

They also both represent extreme opposites in terms of parenting style: Vriska is defined by her tough-love (abusive) spidermom, and the way she's taken on and embodied those traits. Much of her poor treatment of Tavros comes from a misplaced desire to see him rise to the challenge and become "worthy" of her. Meanwhile, Roxy as Mom is overly doting; she's happy to give Rose whatever Rose wants (literally buying her a pony), and we see her magically fix Kanaya's problem by just giving her a new matriorb.

In fact, you could say that Vriska's entire powerset revolves around being able to take things for herself, while Roxy's revolves around being able to end scarcity and distribute things to others without limit. That's not to say that Vriska's self-importance is purely bad: self-importance is also self-confidence, the desire to work hard and earn what you deserve, it's the willingness to pursue higher ideals than mundane, complacent acceptance. Likewise, Roxy's altruistic nature is not purely good: Rose is often very cruel and lacking in empathy (her mocking Dave while Dave is genuinely freaking out about his Bro's weird fetishy puppets and psychological torture comes to mind), and part of that was likely fostered by Momlonde's doting and subsequent lack of consequences. We see the worst of it in Roxy post-retcon, who fixes Kanaya's matriorb situation by just giving her a new one, which means Kanaya never has to reflect on her own actions, what being a Sylph of Space means.