Cryostorm
Monthly Donor
Except the Rhomans aren't exactly fans of the current Vlachian serfdom either, it was put in place primarily to stop Vlachian peasants from leaving and setting up shop in Rhomania and Russia to make farms there. Not to mention the imperial family isn't known for being all that understanding of the plight of the poor rural nobility. Currently it doesn't benefit Rhomania to get involved since that pretty much means declaring war on a long time ally but if the situation ever presented itself I think Rhomania wouldn't mind, especially if it meant getting the chance of putting the king under a more solid despotate-like agreement than the current one and gain the people's support.The Romans wouldn't allow a Vlachian royal to overthrow the nobles at all, as they were the ones that brought Vlachia into this situation in the first place (i.e. serfdom) through their agreement with the nobility. Constantinople depends too much on Vlachian grain and their willingness to buy Roman goods for them to just suddenly end the current status quo. They would be the opposition for any reformist faction in my eyes.
If a Vlachian noble/king wanted to change the current situation in his country and end serfdom, he would have to align with the Latins, since their support would allow them to combat Roman and Russian interests. Of course, that is obviously a huge problem for Romania or Russia, one that will have to be dealt swiftly if Constantinople continues to benefit from the exploitation of the Vlachian people.