To be fair, Dispel Magic a pretty standard vulnerability for animated objects in 5E. It's not just this particular suit of armor.
I don't think it makes a difference. Another witch and a thug will still die swiftly to all of the conjured animals + vrocks/etc. That is, assuming I can pick the spell loadout? I've asked multiple times but not seen an answer so I guess... I'll just run the fight assuming that I get to choose all of the spells on all of the casters, and all of the Celestialock's invocations??? Maybe Kiss will work better than I expect it to on the Vrock(s) and wolves (while PCs bombard with AoEs and Eldritch Blast from the doorway and then duck back behind cover).As written a party of 6 lvl 9 PCs = a Very Strong party and it is recommended that the DM Add another Barovian witch and a thug. Give Esmae another 30 hit points and increase her AC by 1. It takes Esmae 8 rounds to complete the ritual. Our DM nerfed this recommendation some.
Man, the idea of a Vrock who thinks its True Love is a human is... quite funny. I wonder how a Vrock would naturally treat its beloved.