Today I got some help on #vala irc channel (thanks nemequ!), and I must say I am very happy to see Midgard API working with vala bindings. I am also happy cause in this case, there's no need to write bindings in C (as happens with PHP or Python bindings).
First of all, you have to have Midgard2 installed. Then, all you have to do, to have vapi file is to run simple script in vala directory . And finally, you can try example. Note, you must have database created. Midgard.Storage is excluded from vala bindings at this moment.
I hope to see mono bindings soon.