Ohh i thought you meant the WHOLE wasteland. Nuking the I 15 does make the NCR supply line suffer even more then earlier (divide explosion) and nuking Legion inflicts heavy losses on the (huge) Legion camp there, so its true bro.
Yes.
Honestly, the ideal way to go about Yes Man is this.
Make sure the NCR and Legion exist in a form in which they are adequate to fight each other, while you're secretly stockpiling on secret plans and weapons (The Divide warheads, Science from the big empty).
1. Make sure Kimball survives his visit to Hoover Dam. This means, NCR will not oppose your regime. They will blame Kimball and Oliver. "It's his war".
2. Remove the Legion's presence inside the Mojave. Constant raids are even more deadly to the Courier's regime, because for the time being, the towns will have to govern themselves. With that being said, on to 3.
3. Make sure Lanius lives. He will leave Hoover Dam broken, but more reasonable and the Courier realizes that the fight that the Legion has been making might calm down under Lanius as their new Caesar.
4. Don't bomb either of them. If you bomb one of them, the other will gain the advantage over the Mojave.
5. Don't bomb both of them, either, because without NCR, Vegas' economy will falter, and your regime will hardly mean anything.
6. Kill Caesar. As long as he rules the Legion, they have a chance of regrouping post Hoover Dam II and damaging the Mojave. With Lanius in charge, they will exist long enough to hold back the NCR, but in time, they will inevitably end.