The bounded situation described by the RVC and the notion of result bound up with the situation decide the inadmissibility of the RVC in the progressive ‘zhèng zài’ sentence. In fact, a typical Accomplishment verb phrase (an Activity + a quantified object) is incompatible with the progressive ‘zhèngzài’, too, as shown:
Tā zhèng zài xiě sān fēngxìn.
(Int: *He is writing three letters.)
The ungrammaticality of (1) is attributable to the presence of a quantified NP ‘sān fēngxìn’ (three letters), it makes the internal process of the VP bounded. A bounded situation is associated with the perfective, but not with the imperfective aspect, since it is determinate, rather than indeterminate.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.