Let’s take a look, been a minute 
Purple
Zulag
I can confidently tell you, not Zulag. Aside from her niche use defensively in specific formations, she won’t add much to your roster whatsoever. Less defense is your #1 priority right now, but even then, I wouldn’t recommend it.
Quintus/Chakko
The other two have their own benefits for VF (both offensively and defensively). Both Quintus, and Chakko, would pair nicely with your Kunchen; I’d go Quintus more especially with costume, Chakko without. Quintus will hit harder, while Chakko will provide a whole lot more utility; especially if you don’t have C. Sabina, I could see another point going toward Chakko for the immunity to buff skill.
Dependent On…
With how many heroes you now have maxed per color, you can afford to max for more niche settings, like VF, if you’d like - in order to help round out your roster. So depending on your own performance in this area would help determine whether either of these two are worth the mats in the first place. If you don’t spend much, and therefore it’s a good chance these two will be your only options for awhile, you may as well. Especially once you reach 12 tabards (or close)
Although, if you’re happy with your current standings in VF, as well as plan to make pulls in the future and/or barely have 6 tabards, could hold off for awhile and see. Neither of these heroes are stand-outs that demand a rush [on this decision].
Blue
Richard/Fenrir
Richard and Fenrir are out (imo); Richard, even with costume, is essentially a slower and less covering Vela. & Magni or Cobalt can cover most of what Fenrir does, but better (also not a fan of conditional damage tbh, but that’s me).
Isarnia/Misandra
So that leaves Isarnia or Misandra. Imo, both of these heroes are worth maxing, but for entirely different reasons.
Isarnia
As I don’t see a defense down hero in your ice maxed 5* heroes has me leaning toward Isarnia; despite being S1, she still has one of the most powerful defense downs in the game. Making her an all-star vs red (and potentially all) titans, as well as a super-star for rush. 6 turns down is brutal. & Leaves room for her to improve too, if ya ever pull her costume (but even without, the defense down will always give her utility)
Misandra
Then, Misandra, who can be a literal mana pot at fast mana. Maybe less important with heroes like Frosth - for mono - and Ariel - for mana gen - to do something similar, but gaining mana is still gaining mana. She doesn’t always come through, but when she does, it can be game-changing.
However, since Misandra’s role is, and can be, covered [by your existing ice heroes] moreso than Isarnia, I’m leaning toward Isa personally.
Green
Hmmm… this one is tough. I can see a use case for all.
C. Horghall
C. Horghall, again, for VF. Aside from that setting, he probably won’t see much use though, so he’s prob my least favorite.
The Hatter
The Hatter is enticing for the fun alone. Doesn’t pair well with Evelyn, but if you have Almur (and a level 23 mana troop), he can sub in for dispel in a much better way. & For VF?! There’s no better dispeller. + His hit is quite potent. Imo, this one’s the fun one.
Zocc
Zocc’s mana control is invaluable, just as all mana control is. Some love him to death, and swear by him. Personally, I had a really bad experience with him (fired him at MN, and rather than blocking mana, she gained mana and fired, reviving everyone and I lost 
) and since then, I’ve been less enthused lol. Buuut, if you lack a strong 5* mana controller, he is still very worthwhile despite that small draw-back. & Honestly, you can never have enough mana control, even if you already have Hansel, Peters and/or Tettukh in nature. His 5* stats will help him survive a whole lot longer, anywho.
Atomos
& Then, Atomos. Now this hero could sub in as a pretty great tank, as well as having his own use cases offensively. AoE is a lil harder to control, being conditional on mana, but still powerful… with your C. Kad also hitting all, could be a strong pairing. The utility is lacking though, so more for straight offensive bite.
Therefore
I’d probably choose between Zocc and Hatter personally. Despite Atomos’ AoE and revive being enticing, it’s supportive qualities severely lack, and it’s also hard to benefit and/or time correctly. Also, defense (imo) is the least attractive part of the game. However, that’s me… for some tho, he might actually be the right choice [of this group] if damage is above all.
Red
Again, this is gonna depend on how many pulls you do nowadays, and how many mats you have + your standings in VF.
Without costume, it’s difficult to suggest maxing either of these heroes really. I mean, if you have double mats, you may as well for VF alone. But otherwise, what’s the rush? Less the following sounds like something you could use:
Khagan
Khagan is a great hit 3 hero for completing basically all event-style mechanics. Challenge events, NT, MT, etc, especially as he pairs really well with Wilbur and Falcon/C. Marjana. So if that’s a focus for you, and you do try to place, I’d go Khagan
Azlar
Azlar has the tile damage, that could up your green titan scores significantly, as well as improve stack damage. He’d also be a solid map player being AoE. However, you do already have Elena for most of this, so unless you think you still have room to improve, I’d be more inclined to suggest Khagan personally
(That, or wait entirely)
Yellow
So many heroes that excel in VF as options, wow!
Soooo, same rules apply as both Fire and Dark (ie, max if ya think they could help ya improve, or wait til there’s no longer risk of regret)
Justice
Personally, I think Justice is already a super-star for VF. Her blind is insane, and covers all, which can be a life-saver all on its own - especially as she’s difficult to kill. + She pairs very well with Inari in these settings, too. You may find little use for her outside of VF (maybe map stages), but not really having blind at your disposal (Joon doesn’t really count lol), I could see value in maxing her regardless.
Phew, a lot of words
Lemme know if you have any questions!
(Hope this helps 
)
So jelly