Sealing original '71 Overlander roof vents question

[chilipepper]

So after a weekend of rain and camping. I have a few leaks! .. go figure.

After returning home I think the culprit is the roof vents. They appear to have never been sealed since the original factory rubber gasket. This gasket/seal is now very badly cracked and weather checked. So what is the proper way of attacking this? Do I need to drill the rivets and remove, clean add new butyl tape/Vulkem and replace OR can I just clean the surface and Vulkem around them and over the rivets and be done?

I also need to seal the vent by the sink too. I've done the other two but that one is pretty bad!

Thanks for any advice!!