What would happenif you take an expired valtrex (valacyclovir)?

Probably nothing. All medicines degrade with time. Just cause it expired doesn't mean its poisonous. Just won't work as well as unexpired medicine. Actually, it really depends on how old the medicine is. If its relatively soon after the expiration date, i'm sure its still good. All bets are off its over a year past.
It might not work. The life-expectancy of valacyclovir is shorter than that of your herpes infection. It is probably longer than indicated on the bottle, but difficult to know how much so. Generally the rule of thumb is that the expiration date is 50% shorter than it needs to be.