Many manufactures recommend talcum powder/water based lubricant e.g. Ky as lubricant for drysuit seals/ thick neoprene gloves. Since vaseline is cheap and versatile, could I use it for drysuit seal?
It is your drysuit, so you "could" use whatever you want for your seals and zipper.
Personally I would not recommend it, especially with latex seals for the same reason it is not recommended as a sexual lubricant...it breaks down latex condoms and stuff sticks to it making it more difficult to clean up and increasing the risk of problems.
Other reasons why I would not use vaseline (petroleum jelly) are:
1. It is difficult to impossible to remove from fabric, so any of it that gets on the fabric of your suit will absorb and stain the material.
2. Even with good cleaning/surface preparation, it could prevent good bonding of adhesives if/when seals need or zipper needs to be repaired or replaced.
3. It is tenacious and dirt/debris such as sand will cling to the film which can cause deterioration of seals and zipper, can lead to the zipper not sealing properly, and/or irritate your skin if the dirt/debris gets on the surface of your seals that lays against your skin.
Talcum powder/baby powder provided a decent lubricant when donning a suit but does not work when removing the suit when it is wet after a dive. Water-based lubricant can be smeared under the seals after a dive to help slide hands through. It is believed by some that dusting seals with talcum powder/baby powder once the suit is dry creates a barrier that reduces deterioration by ozone.
For zippers there are specific products made by companies like McNett, but they can be expensive. Beeswax and paraffin wax have been the go to products for drysuit wearers in both diving and whitewater kayaking for quite some time. No need to spend lots of money on specialty product unless you want to, as one can just grab a candle or a small chunk of canning wax (paraffin) and gently crayon it into the zipper teeth.
But again, it is your suit, you could use whatever you want.
-Z