I agree with Stifoan. The Cambrige Proficiency proves you can express yourself in English - but this does not mean you can translate well.
A learner with a vocabulary of 2000 words can understand 96% of spoken words but only 80% of average written text. A learner with 6000 words understands 90%. However, using them correctly in context is far more difficult. Like asking an equestrian to ride a tiger.
From Wikipedia, a 1995 study estimated the vocabulary size of first-year college students (high-school educated) at about 12,000, and college-educated speakers at about 17,000 word families.
With the help of a dictionary you can be more precise, but only a native speaker can choose easily between close synonyms. "A Swiss with CPE would be quite good at translating from English into German, but not necessarily the other way around." I'm an English teacher, but no matter how long I learn German, I will always be very wary of translating into it.
Check your vocabulary on
http://my.vocabularysize.com/
Practice on
http://www.freerice.org