Biggles, we could get round the IOW on 70litres in our boat, so I don't think you'll have a major logistical problem. However, I think that your planning is good, as it's obviously highly desirable to keep plenty of fuel in reserve!
It's about 60miles, plus distance to/from your launch point - how many miles to the litre does your boat do? And which way round are you planning to go?
If you are looking for fuel at a good price, then it's probably considering Lymington Yacht Haven
in preference to Yarmouth. Particularly if you buy over 100 litres at once as you then get a further discount (so if you're cruising in company, maybe top up both RIBs at one fill?) When the petrol price went up sharply last year, I filled the RIB up at Lymington Yacht Haven for less than I filled up car, as they have huge tanks and price according to their last delivery.
Ventnor probably isn't worth the hassle for refuelling, unless your engine is particularly uneconomical. As Simon and Tim have pointed out, the berthing arrangements are quite awkward, and transfer of fuel cans likely to be difficult. Distance from Lymington/Yarmouth to Ventnor is under 26Nm.
Portsmouth is the next quayside fuelling point if you're going anti-clockwise, that's a longest leg of 42 Nm. Then the other fuel points are at Cowes and River Hamble.
Bembridge may be a useful option, but I don't know it well enough to recommend it (I think
there's a petrol station there) and access is limited by tide state.