Would anyone know where I can find that part? Or at least get the part number and I could look it up.

Thank you Crew, 

I hope your hubs are not cracked, but just wanted to share a solution I've used to eliminate multiple comebacks after a repair. 

Also, I don't know enough about the engineering or heat specifications of the hubs to answer that, but from my experience hub cracking is more often caused by improper removal or installation of the Vault covers.  There isn't any hard evidence, specifications, or solid reasoning I can offer you, other than direct hands on experience I've had with these types of hub lubrication systems for over a decade and the solutions I've used when having problems with them.  If a new Vault cover and o-ring were used, installed properly, and then fell out a 2nd time, my next step was hub replacement to solve the problem. 

What's his use case?  Always trailering, or is he on the water and it will be in the water all season.  If it's going to live on a trailer, I'd probably go with a bunk, as the scissor ones can be pretty tippy and they take a little more care when driving and securing to the trailer.  If we're talking trailering for putting it in our out for the season, just rent a scissor trailer for each use.  Around here I can rent a trailer for $50 for 3 hours - less crap to store too if you're tight on space.  

19 minutes ago, csleaver said:

When I have experienced Vault hub covers falling off multiple times even after being installed correctly using red 271 (or similar) thread lock, it was usually caused by a crack in the rotor hub.  When the rotor hub gets hot the crack will allow it to expand enough for the Vault cover to fall out.  The crack may be so small that it isn't even really visible.  Replacing the rotor hub has been the most consistently effective solution. 

Wow, I really hope my hubs are not cracked! It would suck to have to now replace those too. What temperatures would be needed to do that?

The only other things I saw from your earlier post and photos that may be an issue were:

The hub nut should be torqued to 18-20 inch pounds, or slightly more than finger tight, not to 20 foot pounds.

The cotter pins were not bent over as recommended which may interfere with the Vault cover install.

The zerk fitting should be removed before installing the Vault covers. 

Here is a link to the Vault service instructions, if it helps.

https://www.dropbox.com/scl/fi/vwsx92z1bwu50wukojrvf/The-Vault-Hub-servicing.pdf?rlkey=dsnrgbs9rfbo4cje6zgzcwkmt&dl=0