I haven't done Vinipata with Rangers so I can't really comment. BG wiki lists it as 50% piercing resistant though. If that's true and then you have to deal with Raksha Stance or Yaksha Stance on top of it, it may be a hard way of doing it if you don't have exceptional Rangers to overcome it.
Vinipata is a fight that hugely favors people who can kill it quickly. I'm not sure the mechanic exactly. It may be attack grows overtime. And the adds of course.
I've done it BLM style and I've done it DD-style. I prefer the DD-style. If you have the BLM dps to drop it in about 3-8 mins BLM it is fine. If not, like you're having with the RNGs the battle gets much harder. (Vinipata more dangerous, more adds, adds needing reslept.)
With DDs it's always been a quick fight for us. Probably under 3-4mins. It does move the pressure to your healers and support since Vinipata can do some really nasty stuff. In particular, Doom which combined with Muddle makes Yagrush valuable.
From a Bard point of view I prefer it cause there's only been 2 Lullabies required. the 2 adds on pop and then a second 2 adds between 30-5% roughly (depends on kill speed). The fights are over before anything woke up or the 3rd set spawned. I wouldn't be surprised it was possible to kill it before the 2nd set of adds with high power 1hrs like mighty strikes.
We use Carols to try and avoid debuffs. Fire in particular for amnesia, Dark for the curse. I usually use Ice aswell if we have the luxury of Daurdablas for the paralyze/bind. I think it can stun too but I don't think I used thunder carols.
It may be worth trying with DDs if that's your stength. No harm trying and seeing how close you come. To be fair, my linkshell has the luxury of many Darudabla/Yagrush/Idris/Aeonics etc. So I can't say how viable it will be for you. It is possible to zerg it down quickly though.
Doom screen, curse screen and Anmesia screen are valuable if you have them. Try to make sure they are up, before you get muddled and especially before it uses Meikyo Shisui.
I wouldn't recommend ES lullaby if it can be avoided. ES lullaby can be partially resisted, especially on a group of targets. Troubadour is much more reliable. Maybe Elemental Seal as a last resort but Random deal/Wild Card/Super Revit.'ing Troubadour back is much more preferable.
Troubadour requiem 7 lasts a long time if you need to sack. Just be aware that Stances can remove it if you are still feeding TP.
All the songs may not strictly be necessary but the way I play Bard I usually go for an over the top style.