I dont unerstand what is supposed to be better about the vault hubs, I never had any issues with greased bearings. One of my vault hubs needs a new seal already its seeping a little. The trailer is a 17 and for sure high miles for a boat trailer but Id think grease would have no issues for 10-15 years.
I have greased hubs using a bearing buddies , The trailer shop advised me the following --- as your towing , the hubs create significant heat , the grease expands and some will almost always leak out , when the grease leaks out , it creates a tiny air void , then boaters submerge the trailer into the lake and a small amount of cold water fills the air gap . Eventually the amount of water increases and ultimately sits at the bottom of the hub . When the trailer isn't being used , that water rusts area's of the bearings and ultimately you end up with bearing failure .
I was told to bring a grease gun with me every trip , before putting the boat into the water , take the grease gun and put a " tiny" amount of grease into the hubs and push out any possible air gap , then put your boat in the water . You have to be extra careful to not overfill the hubs and you could possibly push out the rear seal .
10-15 years ?? I would never go that long , two years or maybe three . I take the I/R gun with me to detect any increase in hub heat --- make sure I don't have premature failure.