I wonder if Single player means "Original Sins game"?

I suggest starting with the original Sins games without either expansion so that you can focus on learning the basics of the game and fleet-to-fleet combat.  Then move up to the Entrenchment expansion where you will learn about starbases and improved defensive structures.  Then go to Diplomacy.

When you've learned about everything in the expansions, then focus on learning each race real well.  Play each race about 25 times to learn their tech trees, ships, strengths and weaknesses.  When you can easily beat Unfair AIs then it might be time to log onto Ironclad Online to start learning how to play against human players, but that's weeks away at this point.

Thanks!

So does the "vanilla" version that came in Trinity also include all the AI updates and other patch-related fixes that came with the two expansions?

I'm assuming it would, but wanted to make sure

While the original Sins probably got a small AI update or two, and the fixes of course, Diplomacy got quite a few enhancements, especially to the AI, unique to it. While this might actually be a good thing for players learning the game because the Diplomacy AI can be a bit unforgiving to newbs, when you want to start playing seriously you should play Diplomacy.

The benefit of playing the original Sins first is that it forces you to master what is the most essential skill of the game, fleet management. Because you don't have strong defensive structures (added in Entrenchment) or many Diplomatic options (added in Diplomacy) to fall back on, your fleets take an even more important role, and against the harder AIs you'll be forced to learn how to manage them effectively or be defeated. Of course, since most people play Diplomacy now this method will add to the learning curve as you will then have to figure out all the new stuff in Diplomacy. So its your choice, learn the things step by step or get everything the game has thrown at you at once.

One last tip I suggest to new players, especially to Diplomacy, is when using a faction for the first time play the game without any AI players (just turn of their slot). This will allow you to experiment with all of their techs and ships without the risk of the AI killing you prematurely, and if you leave the pirates on you can still get quite a bit of military action (for most other situations you may want to play with them off).