I forgot to mention that BEFORE you decide on what to do AND/OR which brand of vinyl you want (assuming that's what you choose) get ahold of the Installation Instruction and the Warranty info.
Yes, improper installation will VOID a Warranty. And most Mfgrs. require a SMOOTH, FLAT surface. In short, preparing the job site MUST figure into the cost of the job, i.e., demolition of the existing siding and (if necessary) the installation of a SMOOTH, FLAT surface. I've seen the wave. Usually, it's due to 'short-cutting' of installing the vinyl over an UNeven surface, or henceforth, where the 'pieces' behind the vinyl have 'fallen off' behind the vinyl.
I know that back-priming DEFINITELY extends the life of the cedar shingles. However, it's the maintenance benefits I like about vinyl (assuming the Install is done properly.)