Sweetening a deal with Visitation rights for seller good idea?

[Trevor Praetsch]

Neighbor to my parents is parting with his vacation home. Plan on buying it and using it mostly as airbnb. Will make offer slightly below asking before it hits MLS. He could most likely get more $ from a bidding war, but to sweeten the deal we plan on giving him and his family 2 weeks a year for free (1 week summer, 1 week fall non-holidays) for maybe 5 years or so. I have talked with seller about this and he has expressed interest. He has been a neighbor to my parents for the past 20 years or so and seems like a real decent guy. He is an older man and is using an agent but wished he wasn't when he found out we were interested.
How common is this situation? Any legal issues I should be aware of? How should this be written up in the offer? Does 5 years sound too long or too little? Any advice is greatly appreciated!