This is a bug, with a workaround.

Assume your Steam wallet is empty. You're on Windows 10, and in SteamVR (wearing an HMD). You are trying to pay with Paypal.

If you go through the steps of trying to buy something with VRC credits and you need to refill, you cannot do it while in VR, unless your steam wallet already has enough funds. In the overlay, you're asked to log in to Paypal, and you can get through most of it. Typing your email, then password is fine. Then, Paypal will try to text you a 2FA code, however, you cannot type in the 2FA code in the overlay. The 6 individual text boxes you see cannot be interacted with from the overlay. You cannot use a physical keyboard to input the code, nor the on screen keyboard. Since the browser/PP auth session is happening in a VR overlay you get stopped at the 5 yard line; you can't interact with the overlay browser from the desktop.

The work around is to leave VR mode, and add funds to your steam wallet from the steam app itself, on the desktop. Then go back to VRChat and buy more credits.

Im reporting it here, because its probably a steam or paypal bug, but a user like me would really get nowhere trying to report a problem like that to those companies. VRChat would probably have better luck and it would probably benefit VRChat to request a fix anyway.