Simply replace the unfilled areas such as "" and "[date]" with your information.
I would like to discuss a recent situation with you. On [date], [name] had questions about my [leave/leave of absence/vacation] and when [he/she] was not able to contact you about my situation, [he/she] chose to discuss it with [one of my coworkers/another employee]. Not only was my privacy breached with this action, but [name] had also relayed some inappropriate personal comments regarding my [leave/leave of absence/vacation].
I was very alarmed to hear about this incident and am deeply concerned by it. I feel that [name's] behavior was both unprofessional and inappropriate, particularly given [his/her] position of confidentiality. If [he/she] was not able to reach you for comment about this matter, then [he/she] should have come to me with it, not to a co-worker of mine. In addition to that, [his/her] personal opinion about it need not have been discussed with my co-worker. I am requesting that you please speak to [him/her] about this situation in order that a reoccurrence may be prevented.
I have always been considerate about taking my [leave/vacation time], never misusing or abusing [them/it]. I always try to schedule far in advance, to make it easier for everyone. I know that you will be able to handle this situation quickly and professionally, and for that I thank you.
Your help with this is greatly appreciated.