Is There a Big Bore Kit for My VT600?

No there is not, and here is why - 

VLX bore & stroke are 3.0" x 2.6" (75mm x 66mm).
Base displacement is 3.14 * (3.0/2)2 * 2.6 *2(# of Cyl) = 36.73
1000cc's = ~61 cubic inches. So, 36.73 / 61 *1000 = 602cc's.
Now, let's open the bore up 0.010.
3.010 x 2.6
3.14 * (3.010/2)2 * 2.6 * 2 = 36.98
36.98 / 61 * 1000 = 606
So, with a 0.010" over-bore, you gained 4cc's… whoopdeefriggin doo!

The obvious reason anyone would consider this kind of stuff is to get more performance out of the engine.
With the VT600 you have the problem of only being able to go so big with the cylinder before you start making those walls suuuuuuper thin! Remember these motors have a water jacket and there’s only so much room in those cylinders!

Now lets not forget, the transmission and clutch on your bike are only able to handle so much.
First, you'll need to beef up the clutch. And then you still can't do jack shit about the gearing. You're just going to wind out the gears much faster and not get the end result you're looking for. Cost vs. ROI (Return on Investment) says you're better off stepping up to a bigger displacement bike. There is no replacement for displacement!

The only realistically applicable reason you would find to bore out an engine like these are for rebuilding purposes. Honda actually offers 2 sizes larger than the stock size for pistons and rings to rebuild a very worn out motor. At some point the ability to improve the machine is not cost effective from a design standpoint. Once you have a solid, reliable package any adjustments from there and your ROI goes down pretty rapidly.

The VT750's had a bore kit made by some company for a hot minute but all i have heard from users was very minimal gains and a big chunk of change sunk into the bike. 

The Honda shadows are pretty solidly designed machines and when you work more on moving fuel and air through them faster, instead of trying to make it a bigger displacement bike, you will get much more bang for your buck and spend a lot less down time, which means more time with your butt in the saddle!