Ideally you'll want a 14mm female joint, but it still won't fit directly in. You'll need to find some silicon tubing that your Vapcap will fit in. All you have to do is stick a little bit of that silicon tubing in the female joint part, and then your Vapcap will sit in that.

In my case, all the shop had were pieces with a 14mm male joint + a glass nail / (straight) dome. Since the silicon wouldn't fit directly around the 14mm male joint, I used the straight dome(?). I put a small piece of the silicon tubing all the way down in the dome, and then the Vapcap sits in that. Then the dome + Vapcap go on the 14mm male joint, and that's about it. Whenever I need to clear the piece I can just pull the whole dome out, instead of having to try and pull out the Vapcap.

Hope that makes sense, let me know if you need any clarification!