I think you ask a reasonable question and there was a discussion on this a while back at Valvetronix.net. The VT+ series are Class D amps that are very efficient, but there are different ways of assessing an amps power/volume output and perhaps the '+' designation is somewhat confusing.
I'm sure the power description/rating can be fully supported by Vox, but when I first tried the VT40+ I must admit the wonderful bit in 'Spinal Tap' about the Marshall amp being '1 louder' because the dial went up to 11' flashed through my mind when I saw the new power selector layout! lol
My best understanding is that the output stage can produce a power rating beyond the model number rating, hence the + in VT20+, VT40+ etc. There is a shaded area on the final output power control. If you set it just on the beginning of the shaded area you’ll get up to the wattage denoted by the model number (40 watts for the VT40+) but if you push the output dial in to the shaded area the amp is capable of producing extra power, although I found there was some loss of clean headroom and general tonality when pushed hard at the top end of the power dial - this might be partly due to speaker limitations of course. The VT80+ reportedly goes up to 120w when the power-selector is fully cranked, but having briefly tried one compared to a VT50 in store I am struggling with that claim because volume wise the VT50 actually seemed (if anything) louder.
I also fully tested out both a VT30 and VT40+ separately (see my you-tube demos) & the VT30 seemed to be if anything perhaps even louder & clearer when pushed than the VT40+. It was definitely louder than the VT40+ when set just to the beginning of the grey zone. I used the VT30 at a rehearsal with a drummer and it had power to spare. However, neither amp were any where close to the volume of my Class A 30w Laney VC30-210 - even allowing for two good quality 10" Jensen C1016Q speakers (versus a single Vox 'stock' 10" speaker), and a bigger cabinet, the volume difference is just colossal.
But cabinet size, construction, materials and speakers are all desperately important and perhaps this is the essential link to your experience because they can have a big impact on volume as well as tone. The VT+ stock speakers will be fine for most people, but a speaker upgrade can really help take an amp to new levels.
For example, my 15w all valve 1x12" class A/B Laney Cub came with a Celestion Rocket 50 (I'd have LOVED an AC15C1 but sadly couldn't afford it : ( ). For a budget speaker it had decent enough tone, but it's only rated 95dB, lacked bottom end, and muddied up when cranked. For a 15w all valve amp (perhaps similar to your thoughts on the VT80+), I was expecting it to have been a bit louder too and wondered how accurate the 15w rating was.
So, I upgraded the speaker to a 100dB 60w Celestion Vintage 30. The tonal difference was very noticeable - bottom end really tightened up, the muddiness when pushed went, and the top end was sparklier & mids punchier. But the difference in volume was HUGE because of the extra 5dB sensitivity.
This has nothing to do with the 60w speaker rating (15w Celestion Alnico Blues are also 100dB which is why they're noticeably louder than the Greenbacks in Vox amps). Take a 50w Marshall with a 12" 100w speaker rated at 101db and compare it to a 97dB speaker in the 100w version of the same amp. The 50w amp will actually be louder because the volume difference between a 50w and 100w amp with the same speaker is only about 3dB.
So, for more volume and clarity from your VT80+ (or any amp) a speaker change is worth considering but I would stress that speakers and tone are very personal and a louder speaker won't necessarily mean better tone to your ears.