vodafone vertrag auf bessere Leitung wechseln?

Well, again someone who thinks Wi-Fi is the Internet..

Wi-Fi is a wireless transmission standard for local networks and has to do with the internet nix per se. Thus, Vodafone is not responsible for your WLAN.

Your problem can therefore be at 2 points:

• The Internet connection (DSL, cable, fiber optic, LTE) is too slow. You can see how much speed your router turns in on its web interface. If you have less than 80% of the speed in your contract, you should contact Vodafone.
• You have badly set up your Wi-Fi. This already starts at the radio link, which should be as free of any hindrance as possible. So place the router and the antennas of the terminals as high as possible. Then it can also be that you use a heavily frequented radio channel. Switching on a less used channel can already work wonders.

Yes, for setting up your Wi-Fi You responsible. Vodafone only provides internet access (DSL, cable, fiber optic, LTE). What happens between your router and the end devices is your thing.