Also found a couple other ways of finding the offerID:
1) Edit another section in the VEC, select with 'Replace with HTML Offer' and select your new offer. Grab the offerID it uses and just delete this edit.
2) In Target, go to the Offers List page and find your offer. Using browser developer tool, if you inspect element of the row your offer is listed, there is a data element there that has the offer id.