There is a bachur that had a stroke, cannot daven on his own, and different bachurim take turns davening with him (standing next to him and being motzi him). I usually do it Friday by Mincha. Question is, is it a neder (I must do it every time) or could I ask someone to do it once in a while, if I have a reason why I don’t want to do it that day (e.g., I want to daven in a shiva house r”l of someone that I owe hakaras hatov to, or I want to daven b’arichus and can’t when I am davening with the bachur)? Is it wrong for me not to do the chesed, or is it Ok if I have a reason (especially since he will get someone else easily)?
You don’t have to be concerned that it is a neder, because it is understood that this type of arrangement will not be permanent, and because we can rely in retrospect, on the mesiras moda’ah that you did erev Rosh Hashana. As far as where you should daven, you can do whichever mitzva you feel is bigger.
R’ Stesman shlit”a, author of Sefer Kol Nidrei