Guys, there is a whole lot of misinformation in this thread and it could land someone in jail.
1) The Texas laws concerning NFA items changed in 2015.
Previously, they were technically illegal and the law allowed for your ATF tax stamp to be used as an "affirmative defense". A defense to prosecution still means you can be arrested, jailed, charged, and have your property confiscated as evidence until your case goes to trial, having to spend big money on lawyers along the way. An affirmative defense only protects you from being convicted, it does NOT protect you from anything up to the point of conviction. Basically, you were guilty until proven innocent.
In 2015, the Texas Senate passed Senate Bill 473, changing the wording of the law to make them fully legal. There is no longer an affirmative defense, because you don't need one, because NFA items are now legal under Texas law.
2) An AR-15 pattern weapon with a pistol buffer (no stock) and a length over 26" is NOT a pistol OR a rifle under Federal law and/or ATF rulings. A pistol must have a length under 26" and a rifle must be able to be shouldered, the described gun does not qualify for either. It then falls under the generic definition of "Title II Firearm".
It's not law, but ATF rules state that a pistol with a vertical foregrip is classified as an "Any Other Weapon" (AOW) and is subject to all the associated NFA regulations. A 26" stock-less AR "firearm" is not a pistol and thus adding a VFG does not constitute an AOW.
Texas law does NOT clarify the definition of a handgun beyond "designed to be fired with one hand" and the criteria for that description has no legal precedent that I can find. Texas is under no obligation to follow or even consider the federal definition for state charges. I wouldn't want to be the first one trying to argue my way around that.
I can envision a scenario in which you get yourself into trouble with the above-described "firearm" configuration, if you put one of the trendy stabilizer braces on it. The braces are explicitly sold as arm braces to allow the disabled to shoot the weapon one-handed, and the DA would definitely argue that makes your 26" AR a "handgun" under Texas law. You could wind up with your weapon considered a "firearm" under Federal law and a "handgun" under Texas law.