There's lots to explore in all 3 countries. Vietnam in particular. There are so many bunkers and forts left over from the war, especially around the Hi-Phan / Hi-Fan pass. A lot of them are just left there beside the road. In Hue in the temple you can see the poc marks from bullets at the back entrance of the temple. The whole temple complex is very interesting, well worth the entry fee. Plus you can take an elephant ride and they treat their elephants well, unlike in Thailand. In Cambodia, Siem Reap is amazing, the temples are incredible, allow for a mimimum of 2 days exploring those. They're not traditional urban exploration, but so worth seeing. Be aware though that there are lots of unexploded ordinance in Cambodia, so don't go wandering off of established tracks. There are some old abandoned hill stations in Cambodia.
As for security, don't go bribing the locals, it is a criminal offense to do so and a criminal offense here to bribe overseas officials. Just don't do it. Most people will not mind you as a tourist photographing weird stuff like abandoned buildings. In Thailand you'll just be dismissed as a crazy tourist, or you'll be asked to leave. Take the hint and walk away. It is better to play it safe than end up in trouble in a country where you don't know the laws or speak the language.