Nope, I am in Singapore and do not have this issue. Are you on stock ROM?
S4 Mini's LTE radio supports:
LTE Band 3 (1,800MHz), Band 7 (2,600MHz), Band 8 (900MHz), Band 20 (800MHz)
Vodafone Portugal supports Band 20 (800MHz), Band 3 (180 MHz) and Band 7 (2600MHz)
So it should work.
I have a somewhat similar problem of owning an s4 mini but only getting HSPA+ (icon "H+" in the status bar) and not 4G LTE (icon "4G"). In fact, since I purchased the unit in late July, I have *never* to my knowledge had 4G service, only HSPA+. The key difference from the OP is that I am in the U.S., have T-Mobile as my carrier, and my s4 mini is an unlocked unit. Has anyone else bought an unlocked s4 mini for the U.S. market and not been able to achieve 4G LTE?
T-Mobile offers 4G LTE in my area (Detroit area), and I have also used the phone in Hawaii, Los Angeles, San Francisco, and Portland, OR. In none of these places have I attained 4G LTE service.
At first, T-Mobile said that the phone probably operates on incompatible bands with their network. But I confirmed that the 4G LTE bands used by T-mobile are supported by the s4 mini. So back to why no 4G LTE??? Any advice would be great appreciated. Love everything about the phone, except for the disappointment of having expected to have 4G LTE on my newly purchased phone.