Did the Vikings practice voluntary human sacrifice?

I believe this may be a reference to the Blood Eagle practice. There is in fact a huge ongoing controversy over whether it was real. For example, one book I have on the Vikings from the 1960s asserts it as common practice (complete with a detailed description), while another I have offhandedly asserts it was made up by Christians.

On the pro side, it is attested in three old sagas: The Orkneyinga, Heimskringla, and Norna-Gests and there's probably a reference to it in The Tale of Ragnar's Sons. Here's the description (translated) from Orkneyinga:

Einarr made them carve an eagle on his back with a sword, and cut the ribs all from the backbone, and draw the lungs there out, and gave him to Odin for the victory he had won...

On the minus side, both of those first two sagas refer to the same incident, so it could well have been a one-off that was thought up on the spot. These particular sagas were not written down until the Medieval period was in full swing. The "blood eagle" isn't attested in any of our oldest and best sources (eg: The Eddas). All of the references we do have came well after Christianization, and Christians had a vested interest in portraying pagans as barbaric. Particularly literate Christians, who tended to be associated with the Catholic Church.

According to Gilbert Poole's Viking Poems on War and Peace, earlier sagas covering the same incident as the first two say that the death in question happened in battle, and only mention an "Eagle" as a carrion bird afterwards. He claims the origin of the story was a mistranslation/misunderstanding of these earlier sagas.

As Yannis says, The "10th-century writer Ahmad ibn Fadlan describes voluntary human sacrifice in his account of a Viking funeral", but note that the designation he uses is "the rus" and it is situated at the Bulghars in the Volga area.

Since what became Russia has its origins when vikings from Scandinavia came to the area, and quickly became slavic speaking and got a slavic identity since they married local girls, at Fadlans time this may or may not be representative of a pagan Scandinavian culture.

So in my view, you should say "A rus funeral", and can add that this was probably vikings.

Note that the word "viking" describes an activity, something you did (they were "gone viking", i.e. travelling for raiding and/or trading), and did not really designate an ethnic group. If you stayed home on the farm you were just a norseman.

Prof. James E. Montgomery of Cambridge has written about this in an article called "IBN FADLAN AND THE RUSIYYAH" published in Journal of Arabic and Islamic Studies 3 (2000).

10th-century writer Ahmad ibn Fadlan describes voluntary human sacrifice in his account of a Viking funeral:

When their chieftain dies, his family ask his slave-girls and slave-boys, “Who among you will die with him?” and some of them reply, “I shall.” Having said this, it becomes incumbent upon the person and it is impossible ever to turn back. Should that person try to, he is not permitted to do so. It is usually slave-girls who make this offer.

I quizzed the interpreter about her actions and he said, “The first time they lifted her, she said, ‘Behold, I see my father and my mother.’ The second time she said, ‘Behold, I see all of my dead kindred, seated.’ The third time she said, ‘Behold, I see my master, seated in Paradise. Paradise is beautiful and verdant. He is accompanied by his men and his male-slaves. He summons me, so bring me to him.’” So they brought her to the ship and she removed two bracelets that she was wearing, handing them to the woman called the “Angel of Death,” the one who was to kill her.

_{Source: Ibn Fadlan and the Rusiyyah}

This isn't exactly the same as the sacrifice shown on the show, but I think Lagertha's appearance during the scene fits the "Angel of Death" description, and perhaps that's a hint the producers were aware of Ibn Fadlan's narrative.

In any case, it's interesting to note that Ibn Fadlan's story may be supported by (sporadic & isolated) archaeological finds:

Ten Viking Age individuals from the northern Norwegian site at Flakstad were analysed for δ13C, δ15N and ancient mitochondrial DNA fragments. The material derives from both single and multiple burials with individuals treated in different ways. The genetic analyses show that the individuals buried together were unlikely to be maternally related, and stable isotope analyses suggest different strata of society. It is, therefore, suggested that slaves may have been offered as grave gifts at Flakstad. A comparison with the remaining population from single graves shows that the presumed slaves had a diet similar to that of the common population, whereas the high status individuals in multiple graves had a diet different from both slaves and the common population. The results provide an insight into the subsistence of different social groups in a Viking Age society, exposing unexpected patterns of living conditions and food distribution.

_{Source: Slaves as burial gifts in Viking Age Norway? Evidence from stable isotope and ancient DNA analyses, Elise Naumanna, Maja Krzewińskab, Anders Götherströmd, Gunilla Erikssond.}