Quick followup for anyone stumbling on this topic:
As of the 2017.9 release, qiime2 supports de novo and closed-reference OTU picking via the q2-vsearch plugin. There's a community tutorial with details:
With this new OTU picking support, it is now possible to perform closed-reference OTU picking with q2-vsearch and then export the feature table for PICRUSt analyses. See this forum topic for details:
