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Unless they've changed it I'm sure there's a free version with limited uploads.
You can also just upload to YouTube - should be able to take mkvs and YouTube has privacy settings that are just as robust as Vimeo.
And yes as above, you could convert them to mp4 or similar via a free tool like Handbrake and send via WeTransfer - they have free accounts with limits up to 2GB per send from memory.
If you've already paid for a Vimeo subscription then remember to cancel but also remember that you likely won't be able to access the videos after the subscription is done. You may also be tied into a minimum period of 12 months. They will also spam the ever living fuck out of you to upgrade to their other plans.
you can make the videos you upload to vimeo, private/ password protected, and then whoever gets the link can download a version at their end.
Otherwise just wetransfer the file over, they get 7days to download and then it's gone..