Given the timeline, it's likely that either the Legion would have a presence there or be very close. NCR aren't that far out.

If you don't want them to encounter any trouble with the Legion, though, it isn't that difficult to avoid them. Though the Legion are brutal, they oddly leave many settlements in their territory alone so long as they follow the Legion's orders. Alternatively, there could be a cool story point involved if any of the characters/NPCs are carrying banned items (such as chems) and are discovered/have to sneak them through.

Also just want to say that this sounds like a fantastic campaign concept for Fallout 2d20.

The Legion in New Vegas is most likely bounded by the Rio Grande, like how there bounded by the Colorado. Since El Paso is on the Rio Grande, the Legion most likely control the city, or atleast the western part. 5-10 years before Fallout: NV, El Paso is either

1. Legion controlled

2. Legion influenced

3. Mexican influence

4. Populated by refugees escaping the Legion

5. Texan/BOS influence

6. Local control

7. Radio-active hell hole

8. Controlled or influenced by remnants of the Master Army

Thanks a lot for the insight.

I will keep you all posted about my plans for the campaign