General:
* Small
* Limited
* Tiny
* Narrow
* Restricted
* Constrained
* Minute
* Compact
Specific to certain contexts:
* Shallow (for depth)
* Sparse (for quantity or density)
* Insignificant (for importance)
* Measly (for size)
* Scanty (for amount)
The best antonym will depend on the specific context in which you are using the word "vast."