As per release notes (http://www.vmware.com/support/developer/vima/vma51/vma_51_relnotes.html) the password complexity requirements for vMA 5.1 are:
The password must have at least:
- Nine characters
- One upper case character
- One lower case character
- One numeral character
- One symbol such as #, $
When setting it up for first time, I also spent some good 10-15 minutes trying to figure out a passwd that would be complex enough for that picky thing. And of course came up with something that I forgot instantly 
Ask google on this and you'll be provided with multiple ways on how to change the vi-admin pass later on to something more human friendly. However if you do so, make sure you account for the risks and security issues, especially if that's your prod environment.
p.s. vMA 5.0 required the same things, except that it would allow 8 characters.
hope this helps
WBR
Imants