EDIT: Apologies. Updated with a link to what gorhill REALLY said:
Manifest v2 uBO will not be automatically replaced by Manifest v3 uBOL[ight]. uBOL is too different from uBO for it to silently replace uBO – you will have to explicitly make a choice as to which extension should replace uBO according to your own prerogatives.
Ultimately whether uBOL is an acceptable alternative to uBO is up to you, it’s not a choice that will be made for you.
Will development of uBO continue? Yes, there are other browsers which are not deprecating Manifest v2, e.g. Firefox.
Do you even need uBO on vivaldi, though? A friend of mine recently had an issue of sites breaking, even with all addons disabled. As we found out, vivaldi already has a built-in adblocker, which uses pretty much the same lists as uBO. In the end it turned out to be one of the easylist’s borked rules…