... after the end of the probation period/time, a maximum of 50% vacation- and christmas bonus resp. (will be paid) voluntary by the company.

... ? . . .

jeweils -> 50% Urlaubsgeld und 50% Weihnachtsgeld

#1: voluntarily

"Resp." really wouldn't be used in a credible English sentence in this manner, nor would "respectively", which is usually a non-idiomatic translation of "jeweils".
Also, although it's a very convenient feature in German, proper English does not (yet) use the hyphen to show an omitted word, as the German does with "Urlaubs- und weihnachtsgeld" nor would English treat "bonus"(or whatever word is in that position)as a singular in such cases.
Of course, it's a bit after-the-fact, but I'd suggest something like the following (written somewhat formally, as this is likely contractual language):
Following the conclusion of the probationary period, the company will pay a maximum of 50% each of the the vacation and Christmas bonuses as ex-gratia payments.