I'm new to X4 and was waiting for a sale on this game. I see the promo is not going to last for long and was wondering as new player if it is worth also buying the X4: Split Vendetta or wait? I can't figure out if that just extra content or if it affects the main game.

It adds one more race, several more sectors and two more factions into the fray. Now we also have the Terran DLC coming soon, announced today. Do you like your games big and have enough computer for it and can afford it, why not buy the collector's edition that includes the first two DLCs.
https://www.gog.com/game/x4_foundations_collectors_edition

Mind you have at least 16GB of RAM and preferrably a processor from the future, if you insist upon 60+ f/s. Oh, and a graphics card with at least 3GB or VRAM.