Answer

These are vows of encouragement (really oaths) to fulfill a commandment (or not to commit a transgression), so the question is not relevant. Even if the vow does not take effect, the prohibition itself remains in force. So what practical difference does it make?
As for your question whether difficulty is grounds for annulment of a vow, I think that at least in this case the answer is no. You described that the person knew in advance that it would be difficult, meaning that nothing has now come to light that he did not know when he made the vow, so there is no opening here for annulment (except perhaps simple regret. But even regarding regret, one must ask what use there is in regretting the vow, since the prohibition exists even without the vow. So straightforwardly, there is not really genuine regret here either).
However, all this depends on the issue of an oath stated in the language of a vow, since in essence this is an oath and not a vow (he is prohibiting an action upon himself, not an object), and I assume he did not swear in God’s name. Moreover, this is an oath to fulfill a commandment, and such an oath does not take effect (a vow can take effect regarding a commandment, but an oath cannot. Nedarim 16). See Nedarim 8a and Shulchan Arukh, Yoreh De’ah, sec. 203:6 regarding an oath of encouragement. It says that he is permitted to swear it out (that is, it is not a false oath), but it is not clear what the status of such an oath is (for example, whether violating it transgresses “he shall not break his word.” Presumably not).