Hi EastendersFan_x!
I'm not too sure that this is authentic, they look rather off to me...

The Zac one looks ridiculous - it looks like a terrible forged autograph. :neutral: Compare to the following
authentic Zac examples:
My TTM autograph from him:
In person autographs from zefron.com:
So, I'm very sure the Zac autograph is fake.
As for Vanessa, a bit harder to tell because I'm not a fan of hers so I don't study her autograph. But, I'd say it's forged too, compare to this preprint:

It looks to broken up in parts, not a flowing autograph as in the above example.

Also, the autograph isn't curly and feminine but rather hard and sharp corners - it is fake in my opinion too.
Hope this helps!
zanne