If you add Flow to MediaWiki-Vagrant, and do no other configuration, you can add topics and posts, but when you later redisplay a topic, e.g. click its permalink icon, you don't see any of its posts! Shahyar, wctaiwan and I all discovered this.
The post does show up in the topic history and the post text is in the flow_revision table, so this is probably a caching issue. Setting $wgFlowUseMemcache = false fixes it (I think it makes Flow use HashBagOStuff). So maybe false should be Flow's default, and overridden by a WMF wmf-config setting.
Another wrinkle is MediaWiki-Vagrant does enable a cache, but it is $wgMainCacheType = 'redis'. So $wgFlowUseMemcache = true should work, since it makes Flow use $wgMemc that ends up being RedisBagOStuff. I'm not sure why it's failing.
Maybe Flow should be flexible and have a $wgFlowCacheType that defaults to CACHE_ANYTHING or CACHE_MEMCACHE, the way system caches like $wgParserCache do. Or it could use $wgMemc all the time, since that misnamed function always does something: it'll use MemcachedPhpBagOStuff or RedisBagOStuff if they're configured, otherwise it falls back to EmptyBagOStuff (dummy cache).