Try Mecafino Banda Azul made by Iberplaca Gelsa its easy to mix and spread on and it doesn't set for hours unlike Yeso which as Nigel Says sets in minutes leaving you with a trowel stuck in a bucket

After you have spread it you can go back over it with a wet float to smooth of any imperfections and it sets really hard

Good luck

Or you could do as Bigyin Suggests and put up some metal studding insulation imbetween the vegas and plasterboard the whole ceiling