The first problem is there's not enough lava. Per a few sources there have been about 6 billion tons/tonnes of solid plastic waste since the mid-20th century; assuming the density of PVC, .05 g/cm^3, my napkin math says this is over 100 cubic kilometers of plastic waste, or enough to build a small volcano on its own.
(edit) You said "ignoring the logistics" so I agree with the other commenter, the outgassing of the burning would be environmentally devastating. Lava doesn't instantly vaporize things: https://www.youtube.com/watch?v=cXJfg_JIUZA