What's happen when you install any add on SF2 series is that some folders are "re-full".
For instance, if (Has I do it) you have selected in the aircraft of your Europe mod folder only the planes you want to fly with or meet in the skies (it means, if you have SF2-Israel, you have remove the kfir and others...), then after having installed SF2-Vietnam, all the planes you have in all the SF2 extensions already installed on your PC will be automatically re-install when you will launch each version of SF2.
What I do in that case:
Before installing a new version, I rename my aircraft folder (i.e. "aircraft_01"). Then, I install the new version, and execute each version. A new aicraft folder is automatically created with all the planes of the game. I quit the game, erase the "aircraft" folder, rename "aircraft_01" to "aircraft". And that's it.
I hope my explanations are clear.