It seems to have become UA policy to offer the DBCFREE certs first and play hardball on the vouchers until it is really necessary. That said, it really varies station to station and flight to flight. If you have a massively oversold flight that is almost certain to lead to cash payouts, they are likely to offer vouchers right away, though some people will still take the free ticket (I don't know why). If the flight is only slightly oversold, you are likely to be stuck with the DBCFREE and hoping that they rebook you into a full-fare class for the extra EQMs.