What's the consequence of not training on vacation?

Elite soccer players did two weeks of high-intensity training (2 min intervals and 30 sec sprints with a -30% reduction in training duration) or inactivity. Before and after, players were tested for repeated high-intensity running capacity (Yo-Yo IR2) and repeated sprint ability (10x 20 m sprints with 15 sec recovery in between).

Two weeks of reduced training duration and high-intensity training led to: Improved repeated sprint and maintained repeated high-intensity running performance. Two weeks of inactivity led to: decreased repeated sprint and decreased repeated high-intensity running performance.

For elite soccer players, as little as 14 days of either high-intensity training or inactivity can change soccer specific physical performance (repeated high-intensity running capacity and repeated sprint ability) for better or for worse. For better: You can become 2% faster within 14 days. For worse: 14 days away from your training schedule will cost you a 29% decrease in physical performance.

Christensen et al. VO2 Kinetics and Performance in Soccer Players after Intense Training and Inactivity. Med. Sci. Sports Exerc. 2011.