I want to exchange my white for greens etc and trhe only place I know where to do this is in the academy with the vulcan officer there who only gives vulcans back in return so I have a load of green, blue and pink duty officers...all vulcan...

Where else can I exchange my duty officers and get different races besides the logical vulcans in my duty list?

Around the back of the building with the personnel officer is an Andorian and Tellarite. You can also go inside and do the same mission with the personnel officer and the race will be random.
The catch is with season 7 it costs a pretty penny in dilithium.

travel through galaxy and you can find in Personal, Duty Officer Exchange Program assignment for different races then those obtained from academy.(only the personnel officer gives random race)
Ex. Alpha Centauri has at least 1 or 2 assignments like this

Have you completed the Paradan and Bajoran assignment chains? If so go to the Cardassian sector and you should see some duty officer exchange programmes, they take about 4hrs each IIRC. Each one is a straight swap, no dil cost included - but there is a small chance you will get a lower quality doff in return.