OBSERVATION : 6 : 4 posts by 3 authors. Attachments (1) 
I observed wasps visit Ervatamia Sp (dwarf, may be hybrid). But their posture say that they are not collecting nectar.

Can someone tell me what they are after ?

wasps are omnivorous. eat insects….  in addition to nectar, fruit juices, honey (stealing from bee”s nests) and food usually consumed by humans.
non-vegetarian prey for wasps are spiders, and caterpillars.
i have seen them hovering around hibiscus plants infested with aphids.
and tabernamontana plants release a milky latex like sap.  may be these wasps like to eat that, or eat insects that eat that. do not know.
30+ minutes of googling did not turn up any such instance, may be the info is not there or is buried in some paper that i did not find.