score:7
The ESTA program is for entry by sea or air (by an approved carrier) only. It is not available for entry by land (or by private charter).
Instead you will need to fill out one of the old paper I-94 forms at the US Border, and ensure that you return the form when you leave. (If you have recently been in the United States, you will resume your previous period of entry instead of starting a new one, and so in that case you will not need to fill out the form.)
As far as I can see, if you enter on a paper I-94 form, you do not technically need to have onward confirmed reservations. The issue is clouded because, if you enter on an ESTA, you certainly do.
However, you will still need to satisfy the US CBP Officer that you have a definite intention to leave within ninety days. Since you have confirmed tickets from Vancouver, this should be sufficient.
There's no need to carry cash, I usually arrive in the United States with only a credit card.
Upvote:3
As long as you have a return flight booked from Vancouver to home you should be fine. The border authorities, both US and Canadian, are concerned principally to know that you have arrangements in place to leave their country. If you've got to go to Vancouver to return home, then clearly you'll have to leave the USA, so they should be satisfied by that.