I flew a couple of weeks ago with Vueling, i needed to bring a car seat but couldnt contact an actual human to ask if it needed to be special baggage (i saw they tweeted that it did which is an extra £80ish quid).
Anyway when i got to the airport and asked, they said it can be sent as a checked bag as long as the length plus width plus height was less than the total size speficied on the website (i think 158cm) so it can be odd shaped/need to go in oversize but you can use as part of your checked luggage allowance.
Mine was a joie 360 which they were happy with (i clicked it into the upright position to make it seem as small as poss!!)