What are the implications of an old Android version?
This question cannot be answered in general. Android is open source and therefore can be modified by the mentioned enterprises. These enterprises will be based on the same "stock | untouched" Android with many of the same bugs and "vulnerabilities".

But:


Nearly all enterprises are modifying Android (interface, core) to match their wishes (e.g. Samsung TouchWiz, Sony XperiaUI, etc.). They are changing they way the software works, causing or eliminating bugs and security problems. So it's possible that some problems are fixed/caused by enterprises themselves. Therefore it's not likely for them to have exactly the same problems.
There is no official list of security flaws available. But there is an official bugtracker: http://code.google.com/p/android/issues/list