What causes an B/S vanguard engine to choke out and throw smoke under pressure of tall grass.

Governor not working properly,
Air filter clogged.
RPMS not fast enough.
Grass build up under deck.
Luck

The choke out and smoke are too rich a mixture of fuel air. As DG suggests check air filter first, that is easy. If that is OK and there is some age on the engine and it has a common bowl carburetor, the float may have lost adjustment from vibration, seen that a few times. Sometimes you can just lean out the high speed mixture screw a little bit to compensate for that saving you disecting the carburetor to check the float.