Upvote:2
In my experience the availability of online check in is hit or miss often for no obvious reason.
For international flights airlines are required to check your documentation (ability to enter the destination country, rules of transit, Covid 19 docs, etc). These documents needs to be verified and the process for this varies greatly. Sometimes you can just type it in, sometimes you need to upload pictures, sometimes it has to be verified by a human. In one case I even had to scan the NFC chip in my passport!
Whether online check in is available depends on what the current rules are, what documents are required, what verification systems are currently available and other unknown factors. Pre-pandemic I flew a lot between the US and China: while online check-in was typically available, sometimes it wasn't (even on the exact same flight where it was available a month earlier). Sometimes you even had to do another doc check at the gate. On our last international multi-segment trip last month we had about 50% success with online check-in travelling with the exact same documentation for each flight.
So the reasons vary all over the place. If Vueling actually refuses to all Romanian citizens, it could be related to either the cost, quality or availability of the Romanian system to verify passports. It could also be just your luck of the draw (or lack thereof).