Because the joints are not sealed, vinyl siding has a very high permeability rating over 40 perms (similar to the wood siding) so it is unlikely to trap moisture long enough to damage the old siding (I assume it is painted).

Another issue is whether to put house wrap behind it which might further slow the drying of the wall to the outside. But we don't know the climate or the kind or amount of insulation so I won't try to guess.

If appropriate for the wall design, you could use a highly permeable house wrap like Tyvek behind the vinyl siding but only if the old siding is painted.

For the vinyl siding to be level it should be installed on a level surface. That can be achieved with reversed shingle or clapboard shims or vertical strapping but the floating vinyl may be more susceptible to damage, but I don't have first hand experience with that. It might be better to remove the old siding. You should also carefully consider how to detail the openings, facias and corner boards.