The U.S. is not an island but we routinely refer to foreign countries as "overseas". People quibble about whether Canada and Mexico are "overseas" as you don't have to go over any seas to get there.
In general, "abroad" and "foreign" refer to other countries regardless of any intervening water. I think "abroad" is relatively rarely used these days. We often simply say "another country". If you're talking about something brought to this country from another country, we say "imported".
Common ways to express your examples:
I'm going to another country for a vacation.
I'm going abroad for a vacation.
Foreign investors brought $1bn into the country last year.
Instead of buying imported clothes, consider supporting local producers.
Instead of buying clothes produced in other countries, ...
I'm suddenly reminded of a news program where the anchorman introduced a reporter's special report by saying, "And now Sally Jones (or whatever her name was) will tell us what she learned when she was a broad ... I mean overseas."