It's not the teachers who are voting - the NAHT are balloting their headteachers, and the NUT are only balloting heads, deputies and assistant heads, because they are the people who must legally administer the tests.
And yes, if the boycott goes ahead, some schools will do them, some schools won't, and some will do a form of internal test. The league tables would, in that case, be a mess next year.
Secondary schools rarely set using SAT levels (they wouldn't know the raw scores, either). Those that do will have to use the teacher assessment, which is supposed to have 50% equal weighting anyway.
Polls are about 50/50 at the moment - the result of the ballot will become clear on 4th May.
I wouldn't like to second guess the likely result - but I do know that the Heads in our local cluster are determined to boycott, including my own headteacher, because they believe it's right for the children.