Think "live performance situation" as you read this.
 
Rather than trigger a sound on a virtual instrument/sampler from "a midi note", I want to trigger a "tempo-synced midi phrase or loop" from "a midi note".

Then the "triggered midi phrase/loop" would play a virtual instrument.

I currently play phrases/loops using a sampler - (by recording the phrase/loop as a wave, load it into a sampler, and assign it to a midi note for playback as a phrase or loop).

If I can instead trigger a "midi phrase/loop" with a single midi note, (which then in turn plays a virtual instrument, then I can accurately "vari the tempo" and/or "change keys without affecting the tempo".

Are there plugins that are "midi phrase/loop players".

Any methods to do this?

In SONAR, the Matrix View and Session Drummer (see http://forum.cakewalk.com/FindPost/3082304 for the most recent write up on how to use) come to mind. Although I believe Live is designed for this type of application.

Try the Matrix View. It can do all sorts of neat things
 
 
Scook beat me again! 
 

+1000 on Matrix View.

Matrix is a good suggestion, but to be more specific, I am looking for a vst plugin that has:
midi phrase/loop creation capability
the phrase/loop can be triggered by a midi note
the vst plugin provides a midi port out (that shows up in Sonar midi port options, and thus can used as the midi input for a virtual instrument track).

Sugar Bytes Thesys.  Quite frequently, Musician's Friend blows it out for 29USD.