RaidenZangetsu

===SPOILER ALERT=== The Vertibird is the most plausible soution to a flying vehicle. I would think the best route would be to set up an NCR quest towards the end of the quest line that would get you an NCR Vertibird like the one the president flies in and then for those evil players do the same late quest for the Legion where maybe they discovered one back east and decide to give to you for your extraordinary deeds of service. And then lastly have one that Mr. House gives or Yes man gives that way no matter what route you take you get the vehicle but not until towards the last parts of the game because something as valuable as the vertibird should not be obtained until late game and you have to choose your side fully before you get one.