Vehicle spawns should scale with the number of players in a lobby. Less players = less vehicles. 99% of low pop lobbies are dominated by vehicles since people can just camp their spawn zones completely uncontested, like the Apache on Paradise. The fact that a team in a low population server isn't 100% guaranteed to have an Engineer (or at least a competent one) makes vehicle domination all the more of a problem, especially when stubborn players refuse to switch class to deal with the threat.