Hi East Coast,
I don’t have any personal experience with either of them so I can’t make any specific comparisons that would be meaningful but there is more general information about the different types of mattress protectors and the pros and cons of each of them in post #89 here. Both of these are knit cotton protectors which would have little effect on the feel of the mattress, are moisture absorbent but not moisture resistant or waterproof, use organic cotton, and would be highly breathable. You would need to contact a knowledgeable retailer to ask them about any additional information they can provide you about each one such as the thickness of the cotton and the current price and then make a “best judgement” decision based on the information they provide you.
A forum search on Vasilo (you can just click the link) will bring up several comments about it from DN who is one of the forum members who owns one and likes it a lot.
I don’t know of any feedback on the forum from anyone that has purchased the Sleeptek protector.
Both of these use high quality materials but I would suspect (and this is only speculation) that the Vasilo would be a little thicker and probably more “protective” and absorbent.
Phoenix