I’ve been watching my VCP day hatch (8" round) deteriorate over that past few years. I’m sure this is similar to many other people’s experience. It has gotten to the point where my day hatch would have a few inches of water in it at the end of a re-entry practice session - so beyond the safety point for general paddling in my opinion.
I plan to try the SEA-LECT replacement, but they are not nearly as readily available as the VCP hatches. I’m working on having one shipped from BC (Canada) to Ontario (Canada). As a side note, if anyone has any tips for closer source let me know.
In the meantime, I spent about an hour today trying out a temporary repair for my worn out cover and decided to share the process. I’ll need to use it for a while to see how it holds up, but so far the results are encouraging enough that some may consider this a somewhat permanent repair option.
On mine the ring is still in pretty good shape but the center was cracked through almost all the way around. If your ring is done this method probably won’t work well enough. The basic concept is to reinforce the center using contact cement and rubber sheet material. I used an old inner tube for the rubber, but if you can find something flatter you can make your life a lot easier. You’ll see what I mean when you look at the one picture showing a really big bubble I couldn’t avoid. Contact cement is a big difficult to work with since you can’t reposition things after sticking them. Once you’ve got things stuck together hit everywhere with a hammer (and a block behind) to get really good adhesion of the glue.
Pictures can be found at various stages by following the link to a shared Google Drive folder:
https://goo.gl/sTK7eI
Keep in mind this is not a tried-and-true repair method, it’s a test. Your mileage may vary. I’ll try to remember to post after using it for some time. Feedback on this and your own attempts are welcome.
Except in an emergency…
…attempting to repair Valley hatch covers is a waste of time. If the center is rotted, the rim will split soon after, probably at the most inopportune time. Just replace it with a Sea-Lect cover and be done with it. I’m in the process of doing that for a half dozen boats.
Waste of time? Not quite.
There are plenty of things in my “waste of time” category. Spending an hour to make the hatch cover “good as new” at least for a short while isn’t one of them.
In my case the day hatch is in considerably worse shape than the others. This means that the ring still looks pretty good, attaching and sealing quite well still. I suspect the difference comes from at least 10x more on/off cycles than the others, with only one hand. Not to mention pushing down with a thumb while pulling the ring up with a few fingers. This puts a lot more stress on it than the two-handed way I usually open the main hatch covers.
I’m not sure I’d have spent a lot more time on it, but it was an hour of time to get the hatch cover to a functional status, giving me time to sort out a replacement. If I could have found the Sea-lect replacement cover at a nearby store I’d have already purchased it and wouldn’t have bothered with either the repair or this thread. Calling it a waste of time is a bit extreme when you consider my motivation. If I was trying to be a cheap bastard and not replace it at all then I’d be wasting my time (and compromising my safety).
Prior to this I had been using a plastic bag wrapped around the cover to keep water out. The solution presented here is much, much more durable. And hey, if it blows out on me I can always go back to the plastic bag until I get the replacement. 
FWIW a Kayaksport Story…
Today I rescued CD Kestrel 120RM that was stored outside in S. Florida for 10yrs. It's faded with a broken seat but the original Kayaksport hatch is in perfect shape..
My point is…
...that you didn't "make it as good as new". The material is compromised and it will fail soon. All you did was buy yourself some time. If that was your intent, fine, but don't imply to others here that it's a durable repair or that it's going to last for long. What you've done is equivalent to building a house on a crumbling foundation. It's going to fail, the only question is when.
You also used water-based contact cement, which I have found to be largely useless. It doesn't bond nearly as well as solvent based contact cements like Weldwood Gel or liquid.