The source of the grapes.
Vintage wine is made with grapes that are all grown or mostly all grown in the same year. The reason for this is that grapes harvested in different years will have different tastes. Using grapes from one year helps to ensure the consistency of the taste.
The aging process.
Vintage wine is carefully aged to the point where the flavors are perfect. The aging process allows the flavors to develop. With vintage wines the aging process is carefully monitored and the wine is carefully handled to ensure proper aging and storage for the perfect final taste.
The bottling process.
Vintage wines are bottled in a single batch. Bottling can alter the taste so when wines are bottled together the tastes are going to be consistent and similar.
Regulation specifications.
In the US there are specific regulations about what wines can be labeled vintage. This helps consumers to ensure that they are getting true vintage wine. In order for a wine to be labeled vintage and with the country of origin 95% of the grapes used in the wine must come from the same year. Without the country of origin on the label, the wine must contain 85% of grapes harvested in the same year.
Regional characteristics.
Grapes from different regions taste different. The different growing conditions produce grapes that have different flavors and that directly relates to the taste of the wine from that region. With vintage wines the grapes all come from the same region. This helps to ensure a pure flavor. In fact, vintage wines are often characterized by the region so this is an important aspect.

