It looks like the more direct translation would be:" To you (all) i am grateful for your help". So, " Io sono grata per il vostro aiuto" to be translated as “I am grateful for your help” would be completely wrong?
I have heard (and therefore said) “ti sono grato” before.
I do not know whether the object is necessary but coming from German (“ich bin Dir dankbar”) it is the natural translation for me.