Tube diameter has negligible effect on light transmission, but a 30mm scope will weigh more. If you want maximum low-light transmission you’re going to need a larger objective and better quality glass/coatings. The main advantage of 30mm is increased adjustment range.
So, the next question is do you like to dial? I’d assume yes given the features of the Razor, but if you’re not opposed to using a reticle you can save some weight. At $1500 I’d be taking a hard look at the Swarovski Z5. For less weight than the Razor you can have a 50mm objective and great glass.
The last big piece for brightness at low light will be magnification. All other things being equal lower magnification will be brighter. The 15x magnification of the LHT in a 42mm obj will give an exit pupil of 2.8mm. So realistically, in low light to maximize usable exit pupil, you’d be restricted to about 7x magnification. Is that sufficient for the shots you want to take?
I’ve never owned a Vortex nor a VX-5, but have had many excellent dealings with PR. If I were you I’d give them a call and trust their advice.