Middle English vauntgard came from Anglo-French vantgarde or avantgarde. The longer form makes the front-position element easier to see.
Vantgarde and avantgarde are alternative forms, not two required historical steps.
Sources
- Merriam-Webster: vanguard
The entry identifies the front-position compound and its alternatives.
- Merriam-Webster: guard
Anglo-French garde is in the guard and defend word family.
- Online Etymology Dictionary: vantage
Also built on *avant* "before": advantage (Old French *avantage*) and its shortening vantage
- Online Etymology Dictionary · vanguard
Meanings of the older forms on the word path.
- Anglo-Norman Dictionary · avantgarde
Meanings of the older forms on the word path.