This is not to invalidate semiotic transmission experiments generally, however a a negative finding may suggest that not all cultural items will change in the process of transmission. This would speak to the trade-off between the set and the individual items within the set: as the set itself becomes compressed it potentially relieves the compression pressure on individual items.
An alternative explanation is that the system is not optimally compressed but that outside sources of inertia are acting on it and preventing compression effects from arising. When writing systems are taught in institutional settings, for example, there is an incentive for standardisation and inertia. After all, it is more economical to teach a classroom of students a single system with the use of common reference materials than multiple variations. The Cherokee script was codified and standardised in print very shortly after its invention and it can be observed that Cherokee graphemes have changed very little over their history.
As far as we know from the historical record a school existed for teaching Vai as early as ca. 1835, but was destroyed in war after a mere 18 months and was never rebuilt (Koelle 1854, Migeod 1909). The next observational evidence for schools is from the 1860s (Creswick 1868) but by 1899 Maurice Delafosse reported that there were no longer any schools and that the script was taught from father to children or through voluntary apprenticeship to a competent scribe. In about the same period, the Vai script was introduced into one Christian school at Robertsport "for the first time" (Massaquoi 1899, 579). As such, it would appear that institutions were never crucial in the transmission of Vai and are therefore unlikely to have exerted too much conservative pressure, however the full extent of institutional mechanisms may simply have escaped notice in the historical record. As Scribner and Cole put it: "We do not know if the script is ever used in bush schools or secret society activities, since we do not have access to information about how these institutions conduct their affairs" (Scribner and Cole 1981). Thus a negative finding in our study may point to historical and ethnographic factors that cannot be recuperated.
 
A major limitation of graphic communication across different time frames is that misunderstandings can never be repaired in on the spot (REF TopICs paper). Thus, it could be the case that the necessity of maintaining a highly consistent and conventional shared code exerts too much inertia on the system, and no significant modification will be permitted. We know from other historical contexts that apparently non-optimal writing systems still manage to be transmitted with high fidelity. Eg, writing systems that have enormous redundancy and graphic complexity (eg, xxx) or those that are borrowed from elsewhere and whose typologies are ill-suited to the phonology of the new language. 
All experimental participants will already be literate in the Roman script and perhaps others. This means that the experiments and the historical data are not directly comparable on the dimension of literacy. Consequently, changes to the Vai input across experimental generations may be conditioned by prior literacy in another linear script, thus items may become more Romanised over time. In itself, this does not undermine the premise since our study is interested in detecting compression regardless how that compression is actualised. 

Concluding remarks and impressions

If Vai is shown to evolve towards a compressible system we may speculate as to how informative this may be for understanding the early evolution of writing. Early Vai certainly shares characteristics of the four origination events we have mentioned earlier: widespread logography with iconicity, variable orientation, seria continua etc.
Graphemes will become more similar to one another. In other words, these items will come to be be generated from the same set of rules, resulting in typographic stereotypes (as for example, ‘stems’ in the Roman script ‘bowls’ and ‘lunettes’ in the Arabic script etc). Within character shapes, any repetitious forms will become abbreviated. In other words, standard repetitions will come to be inferred.

Acknowledgments

The following institutions and individuals helped us secure rare manuscripts for our dataset: Asien-Afrika-Institut (University of Hamburg), Hella Bruns (Max Planck Institute for the Science of Human History, Jena), Valerie Haeder (Library of Congress, Washington). We consulted the Indiana University Liberian Collections, but no dateable and digitised Vai manuscripts were available. The primary Vai data was tabulated by Olena Tykhostup (Freidrich Schiller University, Jena). Lisa Jeschke compiled the German words and sentences used in the Bilerian experiment and Thomas Müller contributed to an earlier draft of it. Michelle O'Reilly helped develop the method for assessing complexity based on anchor points.