TS808 wrote: By the way, I don't see anything on the speaker labels saying that they were made by Celestion...
They're not...the speakers are Vox' own.
The speakers in the AD100VT are distinctly 'average', but even so having played one I just can't agree with your comments of 'unuseability' on certain amp models. My gut feeling tells me that first & foremost you're simply not setting the amp correctly, especially if you are also using an external EQ.
However, if you're unhappy and are convinced that a speaker change is worthwhile then you need to look on various speaker manufacturer sites (eg Celestion, Eminence, Wharfedale, Jensen etc). You'll find detailed descriptions and suggested speakers to fit particular styles, and sometimes you'll also get sound samples (eg Celestion).
Any recommendations from a forum such as this is quite frankly of minimal value because tone is totally subjective. You need to try different speakers to see what YOU like - but even then unless you physically hear them in your amp with your guitar, even that has limitations. This is because the speaker cab itself can have a huge impact on tone.
You also need to think carefully about whether the AD100VT really is for you, and if a speaker upgrade really will make the amount of tonal difference & response you need. There's no doubt that quality speakers can transform an amps tone & response, but finding the right one can be less than simple.
Sorry there's no magic answer - it's simply a case of reading, listening, & experimentation.