Simple question, but does anyone know where I can get some detailed (eg 1:10m scale) vector data from the First World War?
I've been through the archives here, but it seems the best sites for it have already closed.
You could try here. They seem to have a lot of shape files that might fit your description. Though the European dataset is in 30 year increments.
Your best bet is probably going to have to be recreate it. Wikipedia has some WWI maps in PNG format. As others have mentioned, there are plenty of sites with historical raster maps that you could digitize.