When should I obtain a lawyer for slander, defamation of character, or libel?

The statute of limitations for defamation claims in Virginia is only one year, and there is no "discovery rule" that might extend the period. So if the situation is serious as you say, and the statements being made about you are factually incorrect and not just expressions of personal opinion, then the time to speak to a lawyer is now. If a year passes after the statement at issue was made, it will be too late.