I created a vendor lib (Symfony2 Bundle) in git repository. Added composer.json to it.
In multiple projects I use it as a vendor (vcs in composer.json). Everything works fine unless I want to modify original bunlde/vendor.
I can't push to it directly from the projet, that uses the bundle. I have to clone bundle's repo in new dir, change it there and push. This isn't convinient as bundle doesn't work itself, so commits are kinda try-and-see-what-happens.
What would be the best practise to maintain my own vendor repo?
Provided your vendor is installed from source, you can modify and push it. To force installing from source, use the --prefer-source option:
composer install/update --prefer-source
Then cd to your vendor's directory and you'll see it's a separate git repo. You can do a pull in there to get the latest version, make a change and push it back.
The same works for 3rd party bundles. In that case you just need to add a git remote to your fork to push to it.