I would not call 20 Euros generous. Tips are entirely up to the situation and your generosity. Concierges generally do a lot more than just show you the house, and handing 20 Euros is not exactly showing them gratitude or respect, unless you do not count on seeing them again and you only have a few small bags.
When concierges stay in touch, take care of reservations and issues during your stay, and perhaps pick up your luggage upon departure, tips should be much higher. I would say around a 100 for general services, and up from there, all depending on the services, the size of your group, luggage, length of stay, and general satisfaction with your concierge. The type of villa shouldn't even matter, although I believe tips are often in the 4 digits for larger houses.