I received a check for severance and vacation pay. If I cash the check does that preclude me from legal action for unfair dismissal

Accepting and cashing a severance cheque does not in and of itself preclude you from legal action, as long as you did not sign a Release. I would strongly recommend contacting our firm so that you can speak to an employment lawyer who will review your dismissal and ensure that you are getting your full entitlements on termination.