Not sure whether or not I understood the question. Maybe this will
help:

=if((vacation.date-hire.date)<(365*tier2.year),tier1.vac,tier2.vac)

For example,
hire.date = 5/10/2000
tier2.year = 5
tier1.vac = 14
tier2.vac = 21

vacation.date = 5/9/2005 then returns: 14
vacation.date = 5/10/2005 then returns: 21
vacation.date = 5/11/2005 then returns: 21