It's a bit of a hassle, 2-step process, but I use this link to convert google photos shareable links into direct links that will work with vBulletin's [IMG] tags.

The workflow is:
1) From Google photos, click [share icon]->Get Link. Copy the link.
2) Paste the link into the website linked above which will generate a direct link.
3) Copy that link into the IMG tags in your vBulletin post.