There is Project Options -> Code Generation -> Libraries/Paths. I didn't specify any libs, but i specified the path of libosso and also the general vapi path. Seems it doesn't work with the latter. Kinda buggy yet. When i only had libosso in path, Monodevelop showed it in the class browser and i had completion. Now that i have all the libs in the path, class browser works (updates not very fast and i have to click refresh to get the ahole list in the end), but code completion refuses to show up. Maybe some overflow, will test it more.

About the VAPIs - the process of vapi generation is somewhat automated and can be fully so maybe with a list of libs and namespaces. VAPI metadata files can help even more as to define the variable types in the libs, so that they can be easily rebuilt on the next release. The process of generation is described here, although not very detailed. You can get all types of errors there, like duplicate signal and method names for instance

libosso.vapi is available online (in several versions), so patch will be best (i didn't do much). And yes, Vala project probably will benefit from a growing vapi repository.