foreign key is it a real key

Hi,
I have read a few articles and I'm not sure if it's me or the authors but I do
not believe my question was answered.

If I have table that has a PK and a FK - will the planner use the FK just same
as it would use the PK? IOW's is a FK also an index used by the planner?

I have a lagacy table that contains a FK constraint (vendor_id -->
vendor_info). I believe I need an index to improve the performance of a query
because I'm using the column as part of my join. Explain does not appear to
use it as an index (instead of a seq scan it uses a hash join). But the table
all ready has a FK key contraint on the column.

Thanks in advance,

Johnf