I can think of two things although they are related.
Are you using OBR? If you are, you will need to copy your custom vgd file to \PERF-ASSURANCE\vispub\formatting\3.6.20 otherwise it won't know where to pick up the customizations from.
If you have used the macro in your keystroke, you'll need to put a copy of your custom vgd in the same directory. Note that you'll also have issues with portal publishing if you have used this option (things that have been address in the OBR solution).
Although Sandra has already managed to edit an existing vgd file to add a chart it was done by simply trial and error.
I now want to create a new chart from objects in 2 different Hierarchies (I/O & Paging).
The specific chart I want create would have virtual and physical memory shown together (i.e. Mem % & Swap %).
Whilst it would be great if you could answer this specific issue, I would prefer to learn how to do it for myself if possible.
If I am being particularly stupid and have missed the relevant sections in the documentation, then I apologise wholeheartedly, but could you point me to said documentation if it exists.
We currently use Vis 3.6.10 installed on NT Servers and do not use Service Reporting or Reporting Portals etc...
I wish that I could help but editing VGD's is something that I have struggled with myself. It's not that it's that complicated but it's quirky and unpredictable and much of it is hard-coded. There is virtually no documentation that I know of and, between you me and devcon, we have no business releasing this local capability without it. If this bothers you (which I suspect it does) I suggest that you complain by opening a Vantive with support.
Now for the good news; did you know that you can do this this with Perceive? You simply drop the two metrics into a single chart and you're off to the races. If you haven't seen this it is quite cool.
I know that this doesn't answer your question but this is already available in Visualizer. The Node Paging Hierarchy has both Memory% and Swap% so you don't need to re-invent the wheel.
Hope that helps.