that happens with mine sometimes too. mine is 4 years old.
if it's not the whole thing, then don't worry about it. i've always just given some extra water for a couple of days as it seems that the yellowing happens when it's been dry for a bit.
also, if part of the vine is bent enough to break it (even if it doesn't look like it) the part after the bend will yellow and then die. that could be the problem - especially if you did anything with moving any parts of the vine.
