No autovacuum or vacuum causes no user-visible or breaking changes of any kind … it does change the performance of the database backend, but that’s all.
Um...doesn't really answer the question and besides hitting the wrap-around limit causes the server to not start in multi-user mode which is surely a user-visible effect. Performance is a user-visible effect too - the only thing it better not impact is the correctness of the results.
To answer the original question: vacuum's effects are basically immediate so work is not lost if it gets cancelled.
