Probably an internal end of the year commission contest. However, I'm a little confused because Vanguard IS a brokerage. Only a brokerage or similar financial institution (such as a bank) can hold investment accounts, whether taxable or non-taxable.
It could be that they are merely trying to get you to change which funds you are invested in. Tell them to send you all the fund information, including fund fees. Then peruse it at your leisure.
The bigger question is, are they requesting you open a "DISCRETIONARY" trading account?
Because you very, very definitely do not want to do that.
Any account held at a brokerage should ALWAYS be "non-discretionary". This means the client, and ONLY the client, can approve any account activity.
If they try to pressure you into something you don't think you want, tell them:
1) Put it in writing. You are requesting they include any relevant brochures, AND a letter with the broker's signature, stating precisely what service(s) they are trying to sell to you.
2) If they will not agree immediately to send you a letter with brochures, ask to speak to the manager of their department. Insist on it. They will at that point probably back off and cross you off their Call List.
3) If you are transferred to a manager, get his/her full name, his/her exact title, and ask that the phone call be recorded (it is supposed to be, legally). Say that you are feeling pressured by the salesperson and you wish to make it clear you DO NOT appreciate a hard sell.
4) The manager should apologize immediately. Don't get into a long conversation, be firm but polite - cut them off, say thank you, and "I hope we don't have to have this conversation again in the future" and say good-bye, then hang up.