I imagine that Portugal is best annexed before WW2 breaks out?
I won't speak for the current DLC, as I haven't done it in vanilla since LaR came out. But I have some older strategies I'll share.
1) The ol' "Invade Portugal immediately after occupying Spain" trick. Occupy France. Don't form Vichy. Then DOW on Spain (regardless of who's in power). Then hit Portugal. Should take less than three months.
2) Flip Portugal Fascist before the war. Then add them to the Axis upon France falling. (Earlier can cause weird problems with supply German troops and planes there, and you really need to defend Portugal.) For this to work, you need military access from Spain. Not just for moving troops, but because you want to be able to move supply over land to Lisbon.
3) The "Abuse Vichy to naval invade Portugal because the Allies aren't paying attention" strategy. It doesn't work in LaR, because Vichy isn't immediately an Axis power upon formation. But in older patches, Vichy was an Axis power. And it controlled Morocco. So, what I did one MP game was put German and Italian troops in Morocco. Then I moved the German surface fleet down to Casablanca along with a bunch of submarines. Got the required naval superiority and hit Portugal by sea. The British player didn't realize what was happening until too late. This also requires military access from Spain. Again, not for moving troops, but because once you seize Portugal, you need to be able to supply it over land.
None of these strategies are pre-WW2, though.