The consequenses of interrupted vacuum

Very specific question:
Does interrupting a VACUUM (not FULL) rollback all the work it has done
so far, or is the work done on a page by page basis such that at least
some of the pages in the table have been vacuumed?

I'm asking for cases of large tables where autovacuum frequently gets
interrupted. I'm trying to understand if the partial runs are at least
making _some_ progress so the next vacuum has less to do, or if this is
a serious problem that I need to fiddle with tuning to fix.

--
Bill Moran <wmoran@potentialtech.com>