I wash the mud of my waders after every season and that's it. I have 3 pairs of cabela's waders, 2 pairs of which are 5 or 6 seasons old, and none of them have those cracks. I think you may be doing more harm than good with the plastic bag, Vaseline, etc.
I bought a new pair last year when I stepped on some rebar and poked a hole in one of the boots. The pair I replaced was probably 10 years old and didn't have any cracks or leaks.
"God made man, but Samuel Colt made them equal."