I first coded using Markers (with success) then realizing it was no more recommended (especially for clustering).
Having read the examples given in Openlayers doc and this question, I’ve created my Vectors, added some style etc…
But, they are all put in around the LngLat(0,0) instead of their proper coordinates.
You could see below their geometry properties are ok. I don’t know what I’m missing there.

The problem is that while Google Maps projection is EPSG:900913, coordinates of your vector features are in EPSG:4326.
So you have to either specify coordinates in Google Maps projection or use OpenLayers to transform coordinates on the client side. Here’s how you do it:
Once you transformed features you can add layer to the map and it should display features correctly.