I have an infopath form, which includes two date time field to capture research date. But now I have another list which contains the vacation dates. My requirement is that if the user selecting a vacation date, I would like to block the user to save the form with these dates by giving a friendly notification. Can anybody help to achieve this goal? Thanks in advance.
In your scenario, I assume the user enters a start date and an end date. I also assume that you want to check if any date in the time frame between start and end date clashes with a vacation date -- and you store vacation dates in a separate SharePoint list.
In this case, you're out of luck. InfoPath does not have any good or even usable tools/syntax/method/rules for date wrangling.
Handling dates in InfoPath is painful and in most cases will require the use of third party paid solutions.
Don't shoot the messenger. This is probably the only correct answer to your question, even if you don't like what it says.