This vow is rather specifically personal, it seems to me. The vow to forgo th’assay of arms against his majesty is not a vow to never attack Denmark. Just the current king. Which, frankly, is probably all Claudius really cares about. As long as nobody’s invading his country on his watch, he’s probably not too concerned. But that’s a tentative balance – the previous vow only held true with the previous king and so, with every change in leadership, the vows must be newly tested and then renewed again? This is a very precarious peace.