Vesper isn't supposed to have a URL scheme, although they've said one is "coming soon." It has at least a basic one, though, where you can get the app to open. I tried a bunch of different commands to add text, but either the scheme isn't that developed or I need to know more about James Bond's cocktails to use it. Here's what I do now: copy text from Drafts.app and open Vesper. Then I manually paste it in place and add tags.
drafts://x-callback-urlcreate?text=[[draft]]&action={{Copy to Clipboard}}&afterSuccess=Delete&x-success={{vesper://}}
I use this markup system on paper, and am testing out the iOS app. I saw a tweet where someone mentioned a URL scheme, so I wrote one for Drafts. This creates a new task in the Inbox with the full text of the note, then returns to Drafts
dashplus://x-callback-url/addToInbox?text=[[draft]]&x-success={{drafts://}}