I think he's asking whether someone playing an entrenchment program can play someone who is playing a vanilla program.  The answer is "no."  However, I think it's a good point.  With all these new expansions coming out, the online community could become quite fractured, and it would be harder to get a game than it is now.  The solution would be to allow any program (vanilla, expansion, etc) to play any other program.  If you have paid for an expansion, you would be able to use expansion abilities and units.  If you have not, you would not.  In this way there would be one monolithic "community" where everyone could play everyone.

Annatar, had it right. I was asking if after buying Entrenchment, if I would still be able to, in any way, play against someone who does not have Entrenchment, either by how it's programmed, or by launching vanilla sins.  

So after you get Entrenchment, you have the option to just launch regular(vanilla) sins?

Well... don't know about MP but i did a Huge single with 8 Hard AI w/o touching the D-fence Windows and GEEZ..... poor guy that dont have it trying to win.... as soon 1 Full up. SB appear with a Regen Dock.... GL with ur LRM and ur CS blowing up like nothing .

Good. I was wondering about this as well, because I was worried I wouldn't be able to play with my friends who currently have Entrenchment (no, I don't have $10 to spare for good ol' Ironclad )

Note that if you go back and forth between the two, your keybinding settings could get mucked up in some versions.  It seems like I have to reset my keybindings every time I load regular Sins now.

It would be much better for online play if owners of Entrenchment would see vanilla games and be able to play them without relaunching the game.

That will be even more important after the 2nd expansion. There will be four possible combinations of the game with vanilla Sins + 0-2 expansions.