I've joined two kingdoms as vassals (North Empire + Aserai), and both times got nearly instantly awarded either the next or second next fief (both castles, later a town or two). Dunno if they changed this post official release, as I haven't played it after official release, except for a few minutes. What I did, pre-OR, was get the ruler's approval up to at least 40-50 ish. Not sure if that's what does the trick, because I was also clan tier 3 when joining, both times. That could also be a factor.