Welcome!
I am writing a letter. so may I begin it with
'
(I am very)(Very) sorry for disturbing you,
because I know you must be very busy after
your vacation.'
You cannot say "Many sorry". "sorry" is an adjective, so it can only be modified by an adverb, such as 'very'.
Does it make sense? I kind of feel that 'although' is not a proper connection to be used between these two sentences.
1. You have only one sentence, not two.
2. You are right; "although" is not a good conjunction there. 'because' would be suitable there, or you could avoid a conjunction by using punctuation.
a) I am very sorry to disturb you, because I know you must be very busy after your vacation. 
b) I am very sorry to disturb you; I know you must be.......... 
c) I am very sorry to disturb you. I know you must be........... 
I look forward to your advice.
Thank you.
Miao
ps. may I say 'although' is not a proper connection to be used here;
Yes, you can.
or I should have said 'although' is not a proper connection should be used here.
