Whoever you spoke to at Vanguard is misinformed. You are correct that when doing a rollover from an IRA to a 401(k) the first amounts rolled over are the pre-tax funds because the post-tax funds are not permitted to be rolled over to the 401(k). 26 U.S. Code § 402(c)
However, a plan does not have to accept rollovers at all. If they do accept rollovers, recent guidance from the IRS says that the plan can rely on the employee's assertion that the rollover includes no post-tax funds.
