My opinion is that trust is earned. This is true whether you are talking about onsite full-time employees or third-party providers.
I don't think looking over one's shoulder will provide anything other than a false sense of security.
The best thing may be to use a remote control tool that requires a trusted user to 'log in' the outside resource each time they need access. (not 24X7 on demand access) You can record the sessions and review them later, but file transfers and other activities may not stand out in the recording.
The comment about locking out the third-party and forcing them to be onsite is interesting. I am certain that you can find third-party companies that will come onsite, possibly causing their services to be more expensive. The level of service may be lower due to travel time. Remote administration is likely saving you much time and money, realize the benefits of this before you lock them out. Locking them out is a business decision based on risk assessment and comfort level with the third-party. (showing a lack of trust on your part)
You need to protect yourself by ensuring that you can recover any password and lockout any user account used by a vendor. If you do decide to change vendors, the new vendor will need access and the old vendor's accounts need to be disabled.
From my experience in the IT industry, you are much more likely to be a victim of excessive billing hours or 'milking' a project than any type of malicious activity. (think about it from their point of view)
If the administrators should not have access to critical information, don't give it to them. (example: if they are not your database admin, don't give them database access) Try to develop a relationship with your vendor and protect yourself at the same time. Doing business involves risk, assess the risks and realize that they exist. Make them come onsite and pay for their travel is that is a priority. Meeting them at least once may help you to trust them remotely. Know your passwords just in case you need to change vendors.
Someone will need to be able to administer your network. Who do you trust, that is the question.
If security is a priority, involve a security professional or two. This leads to the question, do you trust the security professional. We have no choice in life but to trust other humans. Do you trust your doctor or your airline pilots?