I really want to get the ending with de Vega but I can’t get past the King at the end. I can save the King and get rid of Tomas but when the King starts gathering evidence against me if I do nothing I burn at the stake, if I try and seize the throne I burn at the stake and if I poison him the guards catch me and I burn at the stake. Can anyone help me with what I’m doing wrong?
Sounds like your Subtle stat sucks. Poisoning the king is Subtle and RelKing related, a coup d’etat is much more complicated; you need Subtle, Charm, and usually allies.
Since you’re working with de Vega, I recommend a coup while Tomas is there, without alerting him to your presence. de Vega will almost certainly stand with you against Tomas, as will any of Tomas’ freed slaves. Felix will only do so if you have a sterling reputation (so no hanky-panky until the King is dead).
Worked like a charm! Thanks very much 
For my next playthrough I think I’m gonna work on my subtle stat.