They do not cancel each other.
If you come back from a trip the last workingday of previous roster, the new 28-day roster will look like:
1 day rest
6 off days
3 off days (no rest in between 3 and 6 off days if they come together)
15 vac days (if you asked for them here)
3 workingdays (going in to next roster most probably)
Away about 180 days/year. With €4/hour as F/O from checkin to checkout, plus night-, sunday- and national holiday supplement one will get about €1000-€1500/month as taxfree per diem.