The Southern Vectis Rover tickets are valid for 24 hours from the time on the ticket. I checked the point a few days ago for normal tickets and it is the boarding time that counts: if your ticket says 11.58 you can get on the 12.00 bus at 11.57 and stay on to the end of the journey, but you can’t board at 11.59. (The situation with 11.58 is unclear, but you would be chancing that the ticket machine’s clock is not fast.) i don’t think you can buy them in advance.
If going from Ryde to the Needles Breezer, I think it would be cheapest to buy the Rover+Breezer: if you time it right you can still use it for a bit of the next day. In case it isn’t clear, the £2 fare is for each leg of a journey: if you change buses, you must pay £2 again.
There is a bus information office in Ryde Bus Station, although its hours are only something like 09.00-15.00 with a lunch break. Newport is open 08.00-18.00 with a 12.00-13.00 lunch break.
ADDED 24-HOURS LATER:
I asked in Ryde Esplanade ticket office today and there are still two one-day tickets for use on Islandline: one is Islandline only and the other includes the Steam Railway (only one round trip, I think). I can’t remember which is which (the member of staff had to check as well), but they only sell the ‘combined’ when the Steam Railway is running.
The Needles Breezer should restart on 02 March, but only with an hourly service. Be aware that in windy or similar weather it may be cut back at short notice and not go right down to the New Battery as this may be closed.