Have a look at this article on my blog.
Although Vodafone UK and Vodafone PT are the same company.... the dongles sold each country cannot be used in another country..even if you put a Portuguese SIM card in.
If you have a good 3g mobile signal (as against a "UMTS) signal at your property here in Portugal, you should be able to buy a Vodafone PAYG (as against PAYE:))))) dongle here in Portugal as indicated above. Note you really MUSY check that you can receive a good 3G signal inside the house her in Portugal, otherwise you will be wasting money
The Vodafone PAYG dongle rates can be found here: (in portuguese)
Dongles from Vodafone, TMN and Kanguru are available from the respective shops in any good Algarve Shopping Centre or Town Centre.