there are a few options, but lots will depend on how you are set up in regards to vendor cooperation, your company policy, etc.
This is how I handle this. However, I have a tight environment and maintain absolute control over the translations. Addtionally, I do not require the use of lingo for translation purpsoes. I provide the XLIFF files and as long as their tools can handle them I dont really care what they use. When I get feedback from the in-country personnel and it is integrated into the translation memories, I will inform the vendor that some things were changed. However, when I for example create a project update and send it to the vendor, they only get the new/changed material for translation, so it is not really an issue. As for the changes made, I provide the vendor with the final compiled foreign language project as reference for the new update. I am a firm believer in not handing over translation memories, because its company property. The vendor will only get the updates/new material to translate, but the finals are with the company only. This also allows me to switch vendor at any given time, because I dont have to chase down the TMs from the currenmt vendor for example.
You provide the vendor with the TMX file, but in this manner you basically give the vendor the key to the kingdom so to speak. The TMX file is a valuable asset and the company spent a lot of money on this.
You provide the vendor with the contributor package and have them integrate it if they also use Lingo.
Again, this will largely depend on your setup and how the vendor relations are.