Compared to other sites using the webdip-code vdip and webdip are more connected than other platforms with many players being active on both websites. But while webdip is owned by kestas, the creator of webdiplomacy, vdip is run by Oli.
webdiplomacy is known for its huge amount of members. vdip in return offers many features that are not available at webdip and of course much, much more variants.
Almost every feature on vdip is directly developed by Oli. Since the features are sometimes a bit more experimental, players do not have the guarantee that a new feature always works perfect and it often needs to be patched. If a popular feature was tested long enough, it will be ported to webdip, but there are many helpful additions, e.g. the statistics, that were already developed some years ago and were never taken onto webdip.
If you have cool features in mind, vdip is probably the better website to use since webdip most probably will not upload a completely new and not proven feature. But of course Oli will have to accept it.
Additional features should not change too much of the original code so official updates of the webdip-code are still possible.
The vdip-code as well as the current webdip-code can be found on github where vdip is a fork of webdip. Everything is under the same license, it is free, but changes to the code have to be shared with publicity (if I remember correctly).