I can't find strong sources to back this claim up, although some websites claim that the freed slaves (Americo-Liberians) did subject the local populace to some sort of hierarchical system and possibly forced labor.
First, a probably-obvious caution about that VICE documentary: it's deliberately sensationalist and well worth taking with a grain of salt and looking carefully for further sources of information. The documentary's passing gloss on Liberian history is over-simplistic; that said, the basic narrative of grievance it describes isn't entirely wrong(1).
There was no possibility of the Americo-Liberians "promptly" enslaving the indigenous Africans before the 20th century. The early Republic was basically a thin slice of a few thousand Americo-Liberians and associated freed slaves (glossed in Liberia as "Congoes") who were settled in Liberia after U.S. Navy interceptions of slave ships. Aside from legally and constitutionally abjuring slavery, early Liberia -- no matter what pretensions its inhabitants might have had to superiority over the natives, no matter how it modelled itself on American culture -- simply had no significant leverage on the interior. To whatever extent indigenous traditions of forced labour persisted, they did so independent of Monrovia.
Monrovia did lay claim to the interior in parallel fashion to European colonial powers, and by the 20th century did begin to project power into the interior, especially as the government tried to develop the economy and the rubber industry came on the scene. This was the period of time, from around 1908 on, which saw Monrovia impose Hut Taxes on the interior chieftains, establish the Liberian Frontier Force (known as ill-disciplined and poorly paid brigands for the most part), and saw district commissioners established in the hinterland who established plantations manned by forced labour supplied in part by agreements with the chiefs.
This kind of forced labour system was later used to export workers to Fernando Po and led to a major political scandal in Liberia; but despite that scandal, the system wasn't officially renounced until 1962 and persisted informally right down to the time of Samuel Doe. In fact it likely isn't a stretch to say that Liberia's failure to really address the persistence of the forced labour system directly produced much of the hostility that Samuel Doe came to exploit. So the documentary gets the particulars and timing wrong but the basic source of indigenous Africans' grievance right.
There is a lot of argument going on in comments here about whether what happened in Liberia is according-to-Hoyle slavery. Defined as any system of unfree labour in which the labourer is legally property, most of the Liberian system was not technically slavery, but was the equivalent of corvee labour which in practice could be just as oppressive, especially when harnessed to the demands of globalising capitalism. (King Leopold's atrocities in the Belgian Congo were driven by a similar model of the exploitation of corvee labour.) However, the episode with Fernando Po was essentially a form of labour leasing, which -- just like convict leasing in America -- certainly does meet the definition of slavery. It was not the equivalent of chattel slavery as practised in the American South (the kind of slavery the Americo-Liberians escaped and were offended about being compared to), but there are many forms of slavery which fall short of that maximal example and are still slavery.
(1) This account is based on: Rodriguez, Junius P. The Historical Encyclopedia of World Slavery. Santa Barbara: ABC-CLIO, 1997. P. 410.
The claim in the documentary is that the Americo-Liberians, the term for the ruling class of Liberians of American descent, "immediately" went to Liberia and enslaved the natives using something akin to the chattel slavery system to force them to work on plantations. That is absolutely false. The pictures of plantations that can be viewed on the blog might be related the Firestone Company, which operated slave labor in the 1920's, but Liberia was founded in 1842.
The Americo-Liberians believed they were cultural superior and believed they would "civilize" the other Africans. This ended in many wars with the native peoples, considered "civil wars" since they never took the outlook of racial superiority. (wikipedia.org/wiki/History_of_Liberia)
In the 1920's when they joined the League of Nations, the precursor to the modern United Nations, they were investigated for claims that slavery still existed in Liberia by the presidential challenger of Charles D. B. King. It was found: 1. that the native tribes were still using domestic servitude, which could be considered slavery 2. POW's from the civil wars against the native peoples of Liberia and criminals were being used under an unjust "convict labor system" that should be considered slavery "...Vice President Yancy [of Liberia] and other high officials of the Liberian Government, as well as county superintendents and district commissioners, have given their sanction for compulsory recruitment of labor for road construction, for shipment abroad and other work, by the aid and assistance of the Liberian Frontier Force; and have condoned the utilization of this force for purposes of physical compulsion on road construction for the intimidation of villagers, for the humiliation and degradation of chiefs, of captured natives to the coast, there guarding them till the time of shipment [to Fernando Po and Sao Tome.] The Firestone Company was implicated in this scandal.