I tried the same operation it works for me using build "5638405"(6.5.0).
Same steps like:
1) Copied and unziped the VMware-vSphere-SDK-6.5.0-5638405.zip in C: drive
2) Open command prompt and set JAVAHOME ("set JAVAHOME="C:\Program Files\Java\jdk1.8.0_131")
3) Run the command,
> C:\VMware-vSphere-SDK-6.5.0-5638405\SDK\ssoclient\java\JAXWS> run.bat com.vmware.sso.client.samples.AcquireBearerTokenByUserCredentialSample https://10.10.xx.xx/sts/STSService Administrator@vsphere.local password
Output::
Aquiring a bearer token by using user credentials
Token details:
AssertionId = _6ba49c4d-0fc6-4721-9ee7-79beb6062339
Token type = Bearer
Issued On = 2017-07-03T09:01:32.113Z
Done.
I am thinking it may failed because of some TLS connection, its better to add " -Dhttps.protocols="TLSv1,TLSv1.1,TLSv1.2" " at the end of the command and try.
i.e,
> C:\VMware-vSphere-SDK-6.5.0-5638405\SDK\ssoclient\java\JAXWS> run.bat com.vmware.sso.client.samples.AcquireBearerTokenByUserCredentialSample https://10.10.xx.xx/sts/STSService Administrator@vsphere.local password -Dhttps.protocols="TLSv1,TLSv1.1,TLSv1.2"