libvte2.91 is GTK3, it won't work with GTK2, you need a GTK2 VTE development library. Since you havn't said what system you are using I can't suggest how you get it, or even if its available for your system.
Your answer has already helped! But that was what I feared! I use Slackware-Current. Thanks if you can give me a sim Link to the VTE Gtk2 Library.
And if you know how to answer, is it possible to use both versions in the system? If not, can you tell Geany to read this library from a separate folder for example?