Why is Vagabond on hiatus?
Because its author said, in plain words, that he would not draw it without enthusiasm for it, and the enthusiasm has not come back on schedule.
The shortest honest answer is Inoue's own. In December 2010, after a hiatus for his health, he posted that Vagabond would not return until he regained his "enthusiasm" for the series. It did return, monthly, in 2012, and ran in fits and starts until May 21, 2015, when chapter 327 appeared and the series stopped again, this time without a stated reason or a promised date.
The longer answer is the work itself. Vagabond is drawn in brush and ink at a level of physical realism that Inoue has called exhausting to sustain, against a historical canvas he researches rather than invents. The pauses below are documented at each step. Speculation beyond them is marked as such wherever this archive meets it.
THE PAUSES, IN ORDER
A stop-start history2009-2010 · The ending that kept moving
In April 2009 Inoue expected the end within a year or two; in January 2010, within the year. A health hiatus followed in September 2010, and by December he wrote that Vagabond would not return until he regained his enthusiasm for the series.
2012 · The monthly return
After eighteen months, Vagabond resumed in March 2012, no longer weekly but monthly, at a pace the drawing could sustain. Morning put the comeback on its cover.
2014 · The research break
A hiatus announced as four months, for research, began in February 2014. It lasted nearly a year; the series resumed on January 29, 2015.
2015 · Chapter 327
Five chapters ran in 2015. The last, "The Man Named Tadaoki," appeared on May 21, 2015. None has followed, and none of the five has been collected in a volume.
The work itself
Vagabond's method is the context for all of it: brush-drawn bodies with no speed lines, painted covers, researched history. Inoue has described drawing battle aftermaths as draining, and the Yoshioka chapters in particular as an effort that took real energy out of him.
Not cancelled
The series has never been declared finished or cancelled by Inoue or Kodansha. It stands as an ongoing work on an indefinite hiatus, which is precisely what makes its status a question worth a page.