Nada only goes back to 1995. The retail for the '95 750 Vulcan is $1595.
Being nine-years older than that, it makes this one a "Classic" - hence, the reflection of this fact in the higher price.
That said, Kelley Blue Book sez:
1986 VN750-A2 Vulcan 750
$1995 Retail
$1320 Trade-in
For Comparison:
1995 VN750-A11 Vulcan 750
$2590 Retail
$1745 Trade-in
Both for a bikes in "Good" condition.
NADA is usually the more-accurate, more-realistic (taking their numbers directly from the Dealers and Industry). KBB often under-values and over-values bikes by a LOT, depending on what vehicle they're guessing-at.
This quote worth exactly what you paid for it.