I was going to leave this as a comment but wanted explain a bit more.
The Zulu's and other 'bantu' speaking Africans spread from the Nigeria Basin area to the south of Africa a lot later than what you are wanting.
image from websearch on bantu migration This migration pattern is based off linguistic studies of the South African Bantu Languages. yellow - before 3000BC to 500BC, orange - 500BC to 400AD and red - 400AD to 1100AD.
As you can see, the Bantu people only made it down to the southern reaches of Southern Africa after 1000AD, pushing other people and clans before them. The Zulu's would be found just above the 30degree South Line in circa 1709AD and the strong Zulu Kingdom of Shaka Zulu would only be formed in 1818AD.
There is the Kingdom of 'Mapungubwe' and Great Zimbabwe' at the Limpopo River, at about the 12th Century timeline. Their civilisation fell though, to drought and bad omens a few decades later.
If you are wanting Vikings to travel to South Africa before this 12 Century marker, they would actually encounter the hunter-gathers and other smaller tribes that were being pushed south by the Bantu expansion.
You could have the Vikings do this, create an established settlement and society and then have the Bantu Migration hit them from the North. Instead of melting away like the other tribes did in this timeline, you could have the Vikings halt the Bantu migration or at least hinder it. This would either prevent the rise of the Zulu Kingdom, or possibly allow it to rise earlier with different leaders. One thing is for sure, if there are Vikings in 1200AD...Shaka Zulu type leaders would be needed a lot earlier than 1818!
Also something else to note, The Chinese and other Indian and Asian traders made it to Mapungubwe (or at least the Maputo area, in Mozambique). There is evidence of glass beads and pottery etc. The Swahili language was created as a trade language (though I'm not sure if Chinese influenced it). You could have your Vikings have some interaction with the powerful Chinese Empire before they withdrew into isolation. Maybe even prevent them from withdrawing into isolation.