Vacation

Are these sentences correct?

«Hello Wendy,

Thank you for letting me know. I hope you will enjoy your vacation, which I'm sure is well deserved!»

[Your sentences are correct.  I think the (!) is good if you wanted to add emphasis.  Only one thing - there is no space before the (!), or any other punctuation, including periods and commas.]

Vacation

Are these sentences correct?

«Hello Wendy,

Thank you for letting me know. I hope you will enjoy your vacation, which I'm sure is well deserved.»

I agree that you could use a period instead of an exclamation mark at the end of the last sentece. I would also omit 'will', since it sound more natural without it. Usually when we are giving well-wishes we do so in the present tense, so we would say "I hope you have a great day" or "I hope you enjoy the party" or "I hope you have a good weekend".