You should simply have to turn the neckseal in and pull it down to get a good seal with neoprene. Works fine with mine although when dropping in from a great height you might get a little squirt down the back of your neck.
No silicone as when the zip eventually goes you can't stick the new zip to it.
And definitely no to the vaseline- mineral based oils will rot your seals (in the same way that you can't use vaseline with condoms). Ideally you should use French Chalk, available from industrial suppliers. We buy it here in 1kg bags to keep natural rubber seals in good condition. If you prefer a wet lube then KY or a vet supplies for Vet Lube is what you need. But if you use a wet lube don't use french chalk afterwards as it turns into a white sticky paste. Yum.
Oh- and don't use talc either as if you look on the ingredients it says "parfum" which is basically a smelly oil which will rot your seals.
French Chalk- Cromwell Tools
www.cromwell.co.uk 500g £3.57