There is more about some of the additional risks of a solid surface support system in post #10 here. If you don’t have any additional risk factors you should probably be fine without anything else under the mattress. I also think that the coir would tend to crease and break with constant bending. A wool blanket would also be a good idea if you are concerned.
Post #89 here has more information about the pros and cons of different types of mattress protectors and links to some good sources for each of them.
There is also more information about dust mites and allergies and methods that can be used to control dust mite populations or other allergens in post #2 here and in post #3 here.
There is also more about allergy and bed bug encasements in post #2 here.
This would probably be overkill. If you have a dust mite/bed bug encasement with a wool quilted waterproof protector on top of it (that can be easily removed and washed) there wouldn’t be any need to add a second mattress protector such as the vasilo.