15" of vacuum at what RPM? If it's at 750-800rpm that's exceptional for this camshaft and timing events. If it's at 1200+ not so much!!!
Not NEAR enough initial timing for that camshaft.
Cam may be installed retarded as well complicating matters via the line up the dots method.
DO NOT DRIVE THE CAR AFTER DOING THIS
Start car and get it warmed up, turn distributor so you have 18-20 initial timing and turn the idle speed down. See if it drops any of the timing you just put in. If it does, add it back to the 18-20 range. Car should idle easily at 800 rpm with that camshaft.
Find the most initial timing the starter will handle while cranking OK, no serious dragging or kickback. Subtract that number from the total number you hit and limit the mechanical advance in your distributor.