Vannelli, whose 4-over-par 144 was good enough for medalist honors after the second round of stroke play was completed Monday, beat Larry Walsh 1-up at Bel-Air Country Club.
Ernie Newton, who finished second in stroke play at 5-over-par 145, was also a winner in match play. He bested David Carrera 5 and 3.
Defending champion Kemp Richardson and Steve Bogan, the third-place finishers in stroke play, joined Vannelli and Newton in the winners bracket. Richardson defeated Frank Abbott 3 and 1, while Bogan needed 19 holes to knock off Timothy Miller.
The second and third rounds of match play are scheduled for Tuesday. The quarterfinals and semifinals are slated for Wednesday with the 18-hole final to be played on Thursday.
The Senior Amateur is one of 13 national championships conducted annually by the United States Golf Association, 10 of which are strictly for amateurs.