Goodluck in vRoM or vCoS without a healer or tank You need both.
Same pretty much goes for vWGT and vICP. And vBCII, vCoAII... You know, pretty much all not easy vet dungs
Normal versions can - most of the time - be easily managed by 3DD + healer/tank. But that's if - and only if! - everyone is experienced with mechanics and game in general. When I PUG, I do not want that. If some DD queued as a tank, some other queued as Healer, and here we are, 4 DDs, with 2 of us not champ level... Duh. I can solo FGI, BCI, etc. But II and DLCs? We'll fall apart on Abomination and Inhibitor. So no.
vet FGI is totally ok for 3 DD + tank. As is vBCI.
4 DD is doable, but not easy. With good group you can wreck normal dang in 3 DD + healer/tank and you can also sip tea and smoke in the process XD 4 DD = no smoking XD
English isn't my native, apologies for any mistakes.