It's possible that Vanguard did put a stop at exactly $18,000 of combined contributions. That is exactly what my 401(k) holder, Charles Schwab, did at the time I hit my max each year. It is conceivable that the dollar amounts Vanguard emailed to the OP are the current balances, as in, perhaps he contributed exactly $18,000 and the overage is actually growth and dividends earned over the course of the year. I think this is the most likely explanation, but OP should call Vanguard to confirm. Oh, also, the employer match is allowed to put you over the $18,000 -- that $18,000 cap is specifically for the employee's own contributions.
OP, you could still open an IRA with Vanguard and contribute your money to that, if you'd like. This would not be done through payroll deduction, but would be that you get paid the money from your employer into your regular bank account and then you do an online deposit from there into your Vanguard IRA.