Can you choose a server with vodafone?

I presume you mean the OOKLA speedtest?
To pick a server, just hover your mouse pointer over the geographic location and a label will "pop" up. Click on that label and it will use that server. Otherwise it just does a ping of the nearby servers and picks the one with the quickest ping result.

I just selected Enniskillen and here is the result:



I usually just pick the Dublin Vodafone server as it gives me a consistent result and I can see if there is an issue on my local fibre loop. Using a server outside VF's network may give differing results depending on network load and gateway conditions. You should use a wired rather than a wireless connection as otherwise your speeds are going to vary much more.

Edit - I just had a re-read of your post. You won't improve your actual ping times (for gaming etc.) by choosing a specific speedtest server. The route that Vodofone uses to connect to game servers etc. is determined by their network design and has nothing to do with speedtest.

Yes - I kind of thought that's what you were getting at when I re-read your post. The OOKLA servers are just there to provide the speedtest responses and I think they are hosted by the different ISPs. The route your data takes is predetermined by the ISPs network design, load balancing etc.