Although billed as "moderate difficulty", the Vandals IMHO have an easier time than most other factions except maybe the Sassanids. The reason is that the Vandals can get into a position where they only have to fight 1 enemy for a long time (the WRE), and that enemy's options are limited to predictable courses. Thus, it's possible to stick with a grand plan and get into a strong position before things get crazy, by which time you should be able to deal with the changes.

My grand plan had 3 stages, as follows:

1) Create Festung Iberia and grow strong
2) Create a strong navy
3) Conquer my way along the North African coast and eventually take Rome by amphibious assault.

To do this, I immediately marched west, fighting as little as possible. Every time I crossed a frontier, I got the "transgression" message but nobody wanted to fight 6 full stacks all keeping adjacent to each other. I allied with all the Germanic tribes I met along the way, plus the Huns because they came to me, and only sacked 2 rebel villages (Vicus Vandali and Campus Chattii). Plus I fought some brigands who were in the way, and once to help my friends the Alemanni. This was mostly to get a little money and some experience for my princes. This was all accomplished with negligible loss, thanks to overwhelming numbers of horse archers against pretty much infantry-only armies. Important travel tip: Go through Tribus Chattii instead of Tribus Franki, to avoid a bad ZOC chokepoint that can slow you down several years.

Now you're in the WRE. Turn SSE and walk across Gaul. The WRE won't like you, and won't sign any deals due to all your Germanic friends, but also won't attack your imposing horde. Too bad for them, because now is when you turn on them. Divide into 3 columns of 2 stacks each, each column having at least 1 family member, and sack the following places simultaneously: Massilia, Arles, and Burdigala. This not only makes you tons of money, it creates a nice buffer zone between your main target, Iberia, and the rest of the WRE. Then send each column into Iberia along each of the 3 land routes: around both ends of the mountains and through the pass in the middle. As you do so, build forts to block these routes behind you. The best places are where the path leading to them is only 1 lane wide, but there are at least 2 places on the Iberian side so you can always have more reinforcement stacks than the enemy can bring to the assault. Leave some of your non-horde units in each fort, usually the foot archers, so the garrisons won't vanish when you settle down. Which is what you do now.

As you're building up your new Iberian home, things are fairly predictable. The WRE has no bases close to you, thanks to your devastation of SE Gaul, and now has other problems with various Germanic tribes. Thus, land-based incursions are infrequent and limited in size, so you should have no problem holding them off at the forts in the passes. OTOH, the WRE navy totally surrounds you, so you have to worry about amphibious landings everywhere (and forget getting any money from sea trade due to constant blockades). Fortunately, these too generally don't amount to much, more like suppressing brigands than real invasions, so you should be OK against the WRE. However, it's hard if not impossible to stop the war with the WRE. This really isn't a big deal, because it's not that much of a problem, and you're going to be invading them again in the future. But no matter what you offer them, they won't take a ceasefire. This could be due to some alliances still remaining with Germanic tribes, but it also seems that Vandal diplomats are depressingly prone to being "tactless" or worse, so have 0 or even negative influence ratings.

The really interesting thing about Iberia is that it has 4 provinces and you can only settle in 3 before your horde's all gone. So which one do you leave, and how do you deal with it? It depends on what's going on. You need Cardoba eventually, and it's too big a threat to be left intact with the WRE, but it's too big to be occupied profitably. Best to sack it before you settle anywhere, then let it stay rebel while you hold the other 3. I've sometimes lucked into WRE rebels owning, in which case I've allied with them and taken the other provinces. Eventually, however, Cardoba will revolt back to the WRE, and then you have to deal with it. But by then it's much easier to swallow, and your other cities will be well in the black and pumping out troops.

The rest of the game requires naval might. You at least have to cross Gibralter, even if you satisfy the balance of your victory conditions by marching to Rome overland. However, even just crossing to those straights requires considerable naval might. The WRE has a strong navy and there are TONS of powerful pirate fleets out there, too. Thus, you can't transport and maintain an army to Morocco unless you have a strong navy yourself. Thus, build that up before you make the attempt. Don't just build 1 or 2 ships and go for it, because you won't stand much of a chance.