increases in VO2max, can be seen with a variety of intensities and volume, although to specifically target it you should ride at an intensity that elicits VO2max.
You'd not necessarily be that close using HR based training prescription, as it isn't overly useful for that type of training (the HR lag/response time can cloud your perception of intensity). A power meter is the best instrument for this.
as can be seen in the table towards the end of this linked page, you can see that increases in VO2max may occur from say zone 3 onwards
http://www.cyclingnews.com/fitness/?id=powerstern
ric