Where might I observe people sneaking across the Vietnam-Cambodian border illegally?

According to this article

Ta Mau is believed to be the largest hotspot for smugglers in the Mekong Delta region. Multiple types of products in large numbers are gathered and classified by day at the Vietnam-Cambodia border gate and are piled into boats and smuggled into Vietnam at night.

It sounds like your single protagonist is trying to slip through alone, without being seen at all? One would presume that the best area for this would be as deep in the bush as possible, so far from big cities on either side.

You could take a page from actual news: Montagnard refugee groups are hiding in the jungle along the northern part of the border (Ratanakiri province), and apparently have been doing so (relatively) successfully for at least the last few months. They're fleeing Vietnamese authorities, so it's in the direction you're interested in; though that's not to say that the Cambodian authorities have a positive opinion of them either. Take a look at Radio Free Asia (1, 2) or the Jakarta Post (1) for more information.

You might also want to research North Vietnamese Army and Vietcong tactics during the US-Vietnam war in the 1960s-70s. They regularly crossed over the border in the north to move supplies or forces southward and then back into South Vietnam. This would be more in the vein of finding lesser or un-monitored points and/or bribing border officials, at least for larger movements of materiel.