88% of foreign exchange transactions involve the US dollar. An important reason for this is its use as a vehicle currency: Traders often route trades through the dollar to access better liquidity. We develop a novel methodology to identify dollar cross-trades using high-frequency data from the two primary interdealer platforms. We show theoretically and empirically that cross-trades drive substantial price variation in dollar exchange rates. Using an instrumental variable approach, we find that increased cross-trading causes higher aggregate dollar volatility. This reveals a trade-off: While dollar dominance enhances liquidity, it also exposes the dollar to uncertainty from other currencies.