If I promised to say a certain psalm for an ill person, is it a vow? is it a sin if i miss a day? what if i wrote that i can’t do it on certain days? then it’s not a vow for those days?
If you verbally committed to doing a mitzvah (Is saying Tehillim a Mitzvah??) then it is a vow. But if you said Kol Nidrei on Yom Kippur (and you had in mind that it would be effecive for nullifying your future vows) then you are fine. However the Ramban (I think) says otherwise. He says that if you used the words I vow or I swear then it still counts as a vow and you would have to do Hataras Nedarim. If you specified that you would only do it on certain days then it’s only a vow for those verbally committed days.
If you did not verbally commit to doing it then it does not count unless you:
1)had in mind that you would do it and then you did it once.
2)did it 3 times in a row in which case it becomes a vow.
But if you said Kol Nidrei the above 2 points are not applicable
DISCLAIMER: I think the above is true but then again, I have no clue if I have my facts straight. Ask your LOR to be on the safe side.