You are wrong, Tunnel system for fuel, command center ... is a must have
The Russian made even entrance tunnel systems for their nuke sub in Cam Ranh in 80s , that make sense for a deep water naval base, sub no need to raise up to surface when enter the base. The entrance tunnel is invisible because it's under the water level ... not like what you see in Yulin
About, Rouge Khmer, Vietnam actually counter attacked to aggressive acts of them in the borders.
We once did the same to China ( under Song's dynasty ) in 11th century.
Who is proud of holding hands of the killers like Rouge Khmer ? Even UN is foolish to consider Rouge Khmer as representative of Cambodia ? Vietnam isn't wrong to terminate the mass-killers while Chinese advisors stay behind them.
For your information, Cambodian people are very different to Laos ... They's not loyal to anybody.
Anyway, current Cambodia govt still say Thanks to Vietnam for saving their population under Khmer Rouge rule