The issue here is that you're attempting to get the whole URL and not allowing Rauth to sign the parameters via the Requests' API. This doesn't work because Rauth needs to be able to look at the parameters and sign then in a specific way. Instead you should do this:
print sess.get('http://vimeo.com/api/rest/v2', params={'method': 'vimeo.oauth.checkAccessToken'}).content
<?xml version="1.0" encoding="UTF-8"?>\n<rsp generated_in="0.0044" stat="ok">\n <oauth>\n <token>...</token>\n <permission>delete</permission>\n <user display_name="Max Countryman" id="16760357" username="user16760357"/>\n </oauth>\n</rsp>\n'
Remember that Rauth is Requests but with the addition of convenient OAuth handling. What that means is you should use Rauth as though it were Requests.
The following should work (I personally tested with my Vimeo credentials and it seems to work as expected):
from rauth.session import OAuth1Session
session = OAuth1Session(consumer_key=VIMEO_CLIENTID,
consumer_secret=VIMEO_CLIENTSECRET,
access_token=VIMEO_ACCESSTOKEN,
access_token_secret=VIMEO_ACCESSTOKENSECRET)
response = session.get('http://vimeo.com/api/rest/v2', params={'method': 'vimeo.oauth.checkAccessToken'})
Hope that helps!