As per this posting to ADSM-L by Andy many months ago:
At the time 5.5 VCB support was being developed, the 64-bit VMware VCB framework was not available (though I think one has since been introduced). So TSM support is currently 32-bit Windows 2003.
Good news is that 'backup vm' works on the x64 platform when using the x32 client.
x64 binaries - v22.214.171.124 - C:\progra~1\tivoli\tsm\baclient
x32 binaries - v126.96.36.199 - C:\progra~1\tivoli\tsm\baclient32
C:\progra~1\tivoli\tsm\baclient32.dsmc.exe backup vm -vmlist="fileserver[fileserver_fs]"