I've been putting a lot of thought into this subject, there was a thread running a while back covering the same theme. This summer I will definately be chopping my VX however I have had to modify my dreams and be a bit more realistic than a full targa.

My first point is just sit in your VX and look up. Not more than a few inches infront of your head is the top of the windsheild. Then lean your head back and look at where the door pillar meets the roof. Again this is only a few inches behind your head therefore you'd end up with a very slim targa.
Second to make things look right you have to chop the door frame and create a new window seal against the roof and door pillar. Not easy especially when you consider your roof edge seal will have to be part of your targa pannel.
Having owned 3 Targa top cars: Porsche 911, Toyota MR2 and Honda CRX and knowing the intricate sealing methods used I feel it would be virtually impossible to get a rain tight seal on a "do it yourself" one off basis. (Only the Honda managed to seal in any weather, the Porsche leaked at the mearest hint of moisture)

The options I am currently considering is:
1. A full soft top.
2. Removable pannel that extends beyond the rear door pillar.

In any case I will be removing the rear section as it's just begging for it. I'm currently butchering the Tamiya scale model to see how things would look as well as playing with Photoshop so in a while I'll have some pictures to post.
It's worth bearing in mind I am an engineer so there's every chance I'll trash my VX!!

Col