Here's an example if you know the name of the person to whom you will be writing:
Hello, (name of person).
I would like to take a vacation every 6 months instead of every 12 months. I believe that not only is taking a break from work within this time span beneficial to physical health, but also to mental health as well. Please consider my request and respond at your earliest co
'Hello ___' is not an appropriate greeting for a request made to one's superior. It is too casual. Please note also that the two parts of the correlative conjunction 'not only...but also' must be placed in parallel and as close as feasible to their referents:
I believe that taking a break from work within this time span is beneficial not only to physical health but also
Actually, "Hello" may very well be appropriate. It is commonly used in correspondence at many companies here in California. Perhaps we are a bit more informal, but I like to think of it as being more progressive.
Please keep in mind, SD, that our posters here are worldwide, so that we need to provide globally acceptable solutions as far as we are able, rather than local practice. California is indeed very laid back, but the tone taken in California correspondence may be ill-taken by an Asian supervisor, for instance.
I guess the key here is to know where the OP lives and works. Chinese people, for example, use "Dear" to begin correspondence. I would expect that the OP would know what type of salutation to use and subsequently, replace that which I have written, with it.
As you can see, the poster has difficulty spelling this simplest words and assembling the simplest sentences, so s/he should not be expected to make sociolinguistic decisions. One key to being a useful advisor here is the ability to assess a specific student's level and needs. With more practice, I hope you will be able to develop that.
You'll also note that I asked the OP to make a fir