what are you saying the last vowel sound is?
It depends on what kind of English you speak.
An American 'o' is typically more like an English long 'a' (like in the word 'car')..
The American short 'a' (like in the word 'hat') is quite close to the English English short 'a', although tending slightly towards the English English short 'e'.
The Polish 'a' is roughly mid-way between both the English English and American English long and short 'a' sounds.
The accent in Potlatch, somewhere in the US - I really don't know, but that would be more relevent to the questioner here.
I imagine if you tell an American that the 'a' in 'Wrocław' is like an English 'a', they might interpret that as something that sounds more like a Polish 'e'.
Have I confused matters enough yet, or shall I go on?