I'm in the middle of a play through of Vanilla BG1, will installing the widescreen mod break my save?

No.
But once it's installed, & you have saved a game with the new resolution, you can't change the resolution again without starting a new game.

Be aware if you intend to also use this GUI mod, you will be limited to the resolutions described here.
http://athkatla.cob-bg.pl/viewtopic.php?t=3257

Please ignore the above post.
There is nothing stopping you from going back to a save from before you installed the widescreen mod.
The only thing you can't do is remove the mod or change the resolution after you have saved your game with it installed.

And this person knows nothing. The widescreen mod alters game assets, and those altered game assets are stored inside your saved games. The altered game assets will not be compatible with anything other than the resolution that the WS mod uses. Ignore my advice at your peril; ignore this person if you want peace of mind.

This is not in dispute, and exactly why I said: But how does that stop you from using your previous saves, either with or without the widescreen mod installed?
Oh, and in case you missed it, that was a rhetorical question & I'm not interested in your silly reply.

No, that's why you bothered to quote me. Duh!