The victory gives Vanguard the No. 1 seed in the eight-team conference tournament that begins Wednesday.
Vanguard's only other GSAC regular-season title was a co-championship in 2004.
Cornwall's two goals, in the 27th and 68th minutes, give him a team-best 13 for the season.
Seniors Victor Tapia and Tomislav Colic both added goals, while senior Trevor Herrera had three saves for his sixth shutout of the season.
Vanguard, which announced on Oct. 6 that it had forfeited five nonconference wins and one tie due to the use of an ineligible player, has won seven straight since a 1-0 loss at Cal Baptist on Oct. 5.The Lions were ranked No. 5 in the NAIA before announcing the forfeits.
Coach Randy Dodge's Lions have outscored teams, 16-1, during the winning streak and outscored foes, 25-5, in GSAC play and 48-11 this season.
Alejandro Lopez and Kyle O'Brien had assists for Vanguard, which has eight shutouts this season.
Westmont fell to 7-7-3, 4-5-1.
Big West Championship
RIVERSIDE — The UC Irvine men's and women's cross country teams finished sixth in the Big West Conference Championship Saturday at UC Riverside.
UCI senior Colin Hacker was seventh in the men's race to earn all-conference honors, finishing the 8,000-meter course in 24 minutes, 31.7 seconds.
Colleen Lillig paced the Anteaters women, finishing 23rd over the 6,000-meter course in 21:29.5. She was the top freshman in the conference.
Brett Matejka (17th in 24:48.7) and Steven Melena (24th in 25:02.1) were also in the top 25 for the UCI men.
Katherine Nielsen (26th in 21:43.5), Melissa Mead, a Newport Harbor High product (33rd in 21:56.3), and Chloe Fata (34th in 21:57.1) were additional women's contributors for UCI.
UC Davis won the men's title and UC Santa Barbara captured the women's crown.
Jon Peterson (UC Davis) was the men's individual champion, while Amanda Moreno (UCSB) topped the women's field.
MEN'S WATER POLO
Cal 18, UC Irvine 9