I have always traveled with my labs and bad guys have never messed with our van.
Getting older and needing potty stops on those day trips I feel very comfortable leaving King Vanity in the van. He asks that I park where he can see me.
I don't have any seats in the van except for the drivers. When I put down the lift to load or unload the chair the boyz are right there, well one now as JC is no longer with us and the two pups are still traveling in crates.
The store carry outs always enjoy visiting with the pups and if we are doing something near the van I leave the back doors open for them to socialize.
Bottom line though bad guys don't know that their bark doesn't mean they bite.
I'd personally be more fearful of your Siamese Cats then your Lab.
My big 14 pound Blue Point male has actually bitten my son (who foolishly tried to intervene in a dog-and-cat spat by grabbing the cat rather than throwing a blanket or a glass of water over the conflict). He also chomped my husband one time (again, trying to intervene in a fight between the cat and another cat . . . the boys just don't THINK, not the cat boy nor the human boys . . . . he knows better than to bite ME and never has . . . )
My Lab has never bitten anything but her dinner and her chew toys. She doesn't even bite down on the ducks she retrieves . . .