From the recent Vodya post, I see most people wanting an ATK buffer, which I completely understand and agree with if we're talking at a c0 standpoint.

However I believe this doesn't hold true anymore if one decides to invest vertically, because : 

* Skirk R1 gives her 48% ATK on top of being very high base ATK
* Escoffier R1 gives skirk 32% ATK
* Skirk C2 gives 70% ATK (albeit only for NA playstyle)

\-> Which already equates to +150% ATK and that's not even accounting for ttds, totm & noblesse which, all together, give +238% ATK, a ridiculous amount.

This comes to a point where my C3 Skirk, who has 2476 ATK on her profile card, has more than 5100 ATK when she's on-field with all these buffs effective.  
So I feel like if Vodya turns out to be an ATK buffer it wouldn't help much, atleast from a vertical investment standpoint.

An ATK speed buffer would be "good" but also kill a good portion of the fun of playing Skirk for me : her combos. Had you already tried to do N2D combos with Dahlia as the 4th slot, you know it's not enjoyable at all and the difference is very negligeable anyway since more ATK speed reduce the value of combos over spamming as the time during a dash cancel doesn't get reduced by ATK speed obviously.  
So unironically, I hope she does not buff ATK speed (+ this doesn't buff at all the burst playstyle which I think is a valuable aspect of Skirk as well, being able to condensate her dmg into a short window rather than a full lenghty rotation).

So my question is : from a vertical investment standpoint, what stats would you REALLY want Vodya to give ? Appart from def shred which would be by an enormous margin the biggest improvement for the team, what else ?

I think that a "vulnerability" type of effect, such as the one we have on HSR, would be amazing. Because the thing is that at high investment, all stats are already saturated af : 

* \+5000 ATK already at C2
* \+310% c.dmg with Esco C1 (+60%) and good build (+210%) + azurelight (+40%)
* \+277% dmg with Furina c3 (+124%) / galleries (+75%) / cryo goblet (+46%) / miniburst (+20%) / scroll (+12%). 

  
\-> We lack another form of multiplier

Either this or have CRAZY high quills with good uptime, not like the 5 quills we get from Escoffier C2 or the ridiculous small quills Shenhe gives.   
I'm talking about something like Lauma C2 or Xianyun C2 level of quills, which are like the double of what Escoffier C2 gives and I want that to have no limited amount (or atleast a very high amount), that could also adapt the dmg it gives based of if we're hitting for normal attack dmg of burst dmg.

I'd also quickly want to address the support sets : they are terribly bad for the modern standard.   
\+20% ATK from totm compared to the +40% dmg from hex/nightsoul scroll sets, or what the lunar/stellar set provide to their carries, is night & day let's be honest. 

Having Escoffier bis set being a ridiculous +12% dmg while not even giving anything on the 2p just screams to me that new sets are needed for this team to take a new step forward, alas Hoyo will probably never put new sets for old units so we're probably stuck with totm/noblesse and hopefully Vodya on a new modern op set.

Quite a lot of yapping but I'm genuinely on the fence that an ATK buffer is a received idea that wouldn't benefit Skirk much in the long term, especially if one wants to invest more into the team.  
Def shred is almost a necessity, but even that alone (unless absurd amount) wouldn't solve the gap between her and the likes of lunar/mavuika/stellar.

For C2+ Skirk havers : don't you feel like an ATK buffer as the 4th slot would be a "false good idea" ?