i have an 18' zx250 sitting on a skeeter trailer w/vault oil bath hubs. i have yet to remove the tire/hub to check, but can i replace these hubs with the older style 1 3/8 x 1 1/16 1750# hubs?, or are the older style not compatible with my trailer's spindle/axle?

I guess you would need to measure the spindle to see what size it is... What I would do, is to use your existing hubs. A hub is a hub... Clean or replace the bearings and races ( if they look ok, then reuse them) Repack bearings and replace seals. I've done several this way. Vault hubs are nothing different. They are just a hub!

You can just use the same hub - pack the inside bearing real good with new seal, use the grease fitting in the end of the axle to put grease in until is starts showing out the outside bearing (spin the hub every once in a while while you are putting grease in) , you will need to get either a cap or a bearing buddy to use on that axle. I just used the red & Sticky grease at Lowes. Changed one axle 3 years ago and no issues at all.

What the others said. Just need the new bearing caps and seals. I know grease seals won't work with oil bath but not sure about vice versa. Just Google it or call a trailer parts place. There is lot info out there on converting oil bath hubs to grease. I recently replaced the bearing caps on my oil bath hubs with the ones that have the fill plug on the bearing cap and saw a lot of talk on converting to grease.

I thought about the seal issue when I converted one axle to grease - I'm certainly no "seal" expert but seemed to me that since the regular vault grease is so much thinner than regular grease the seal for vault grease should be better at sealing.