I want to try a more expensive film like tri x to see the difference and also to see if this developer is really the problem. Because I am only paying 5.99 to develop and scan
Tri-X is b/w film developed in b/w chemicals at room temperature, Vista is colour negative film is developed in C41 chems at 38 degrees, and probably by a machine at Asda, so if you want to compare films souped by the same processor, best stick to colour neg, you could try Asda with some Porta.
But before you do that, you ought to consider what you are comparing.
You don't know whether your Vista was correctly exposed, and you won't know the status of the Porta either.
You don't know the condition of the chems in Asdas machine, they may have been old for the Vista and be new for the Porta.
You don't know the competency of either the processor nor the scanner operator.
You don't know what that operator did to recover (or not) detail lost as a result of under/over exposure or processing errors during scanning.
In short, you have little control over what you get, Asda scans are probably low res and don't give you much file content with which to correct them yourself.
So you either have to take control by developing and scanning yourself, or......not worry too much about what comes back, it is what it is and don't be too disappointed if occasionally you get iffy pictures.
I think using good quality film is the way to go but if you hand over control to somebody else, there is no guarantee you'll like what they produce, there is no guarantee a roll of "expensive" Porta will come back looking better than Poundland Vista.
If its any consolation, I'd get C41 deved at Asda no problem, and not worry too much about the negs, but I'd have to do the scanning myself, that's troubleshooting by inspection, at least then I could decide if I'd cocked up, or Asda did.