The standard 'new' VX800 rear disc is 6.0mm thick +/- 0.2mm. The wear limit is 5.5mm.
The scoring on your disc may be a problem with the type of pads that have been used that has lead to the scoring, or pads have bee run down 'too low' at some stage.
I've never had an issue with my rear disc in general terms - but did once have a problem when a small road chipping got lodged in the rear brake pivot and held the rear brake partially on, This had occurred on a freshly chipped road, & the chance of it ever happening again would be slim. Clearly any form of brake binding has the potential to ruin the brakes performance, but that is through over-heating, which commonly leads to a distortion problem (though prolonged overheating and cooling could lead to the disc becoming susceptible to scoring - but only on a poorly maintained bike - i.e. one with sticking brakes).
Fortunately when the chipping incident occurred I was riding with a couple of mates and one of them asked me if I was having a problem with my rear brake as the brake light was staying illuminated. This was spotted quickly & there was no long term 'damage' to the rear brake.
As a precaution I did change the brake fluid as any excess temperature could have 'spoiled' it - this was despite the fact that I had changed the rear brake fluid around 6 months earlier.
Many people neglect to change their brake fluid, and this is not good as old fluid will contain some water moisture and this softens the feel of your brakes, plus having 'water' in the system can lead to internal corrosion of you braking components, leading to sticking callipers or cylinders etc. Brake fluid should be regarded as analogous to oil in an engine, and needs to be replaced at regular intervals (No longer than 2 years).
So summarising - I'm not aware of a significant problem with the rear disc brake on the VX800.
If your disc is over 5.5mm thick it is classed as serviceable, and putting on a new set of pads might help - but if the scoring is excessive, then the pad life will of course be less than normal. That is your cheapest option.
If you are patient you might find a better second hand disc - but breakers seem to charge around £50 for them, and what they are asking for them is no guarantee of how 'good' the disc is. Some breakers charge OTT prices for worn parts - but will offer a refund if you are not happy with the part when you receive it (i.e. it is your decision to make up your mind how serviceable the part is - not theirs). You should fit new pads to any replacement disc.
If you are 'anal' about the condition of your rear disc (that doesn't sound right
), then the only way you will be satisfied (here we go again
) is to replace it with a new item. You could be lucky and find a genuine Suzuki part offered cheaply as 'old stock', or you could use a decent aftermarket one such as from EBC (You will be talking a 3 figure number for that).
Whichever option you chose - if you are not sure when it was last done - CHANGE THE BRAKE FLUID
Hope that might help !