Can I be replaced by replaced by an INS sponsored employee?

I was recently let go for reasons of re-organization and downsize. The position in this area will be filled by a foreigner who is being sponsored by the employer for a work visa. I also have vacation bundled into a severance package pending signature. They are extending a few more days on top of my vacation pay as severance. My questions:
* Can My job be replaced by a a foreigner? (Maybe this is an immigration question)
* Can they hold my vacation pay pending my acceptance to a separation agreement?

My biggest issue, what burns me is the excuse or downsize or re-org knowing they will be sponsoring someone to move into the area from out of the country probably for less pay and for the same relative position.