The passed vague time of a decade
The passed - We're no longer in the 90's anymore. It's 2001!! So the timeline 199X covered has passed.
vague - 199X. Theres really no such thing. X often acts as variable (Thats any number to you algebra dropouts like me). And because this is in a timeline, the only number X can be is 0 through 9. So, it's nonspecific, thus it's vague.
time of a decade - 199X covers an entire decade; 10 years.