vaal molten shell vs molten shell

what is the diference between the 2 gems?

can i use vaal molten shell with cast when dmg taken? (low level gem)

Vaal molten shell explodes with every hit.
So lets say you are surrounded by 10 enemies and each is hitting you every second for 100 damage every second.

Every 1 second the shell would explode 10 times until duration runs out or the shell absorbs the amount of damage it can.

Also there is no point in using a low level vaal shell imo, you can't spam it.

TY

any support gems that goes well with vaal moltenshell and firestorm?

I imagine things like Vaal MS > Conc. Effect > Chance to Ignire > Life leech would work well with it.

Or really any combo of these:
concentrated effect
life leech
fire penetration
chance to ignite
empower
increased area of effect
iron will
crit damage
crit chance

What you use is up to you on the end.

Keep in mind you need to charge it up though, so you can't just run around with it on all the time.