might try a shop vac with small long wand, made from like 1 inch pvc
pipe to suck the loose fill insulation from the vents.

have you tried accessing it from outside? might be able to vacuum from
outside too, or blow topwards inside

if you install a fan with open ridge vents it wouldnt do much sucking
air from nearby ridge vents.

if you choose fan have ridge vent removed from above, tar paper over
the holes and replace shingles

i would try getting the soffit vents open