My scroll wheel also does not respond to any settings changes when using Vegas .. My mouse zooms all the way in about 4 spins of the scroll wheel .. but .. there are other methods.
CTRL + UP/DOWN arrow will get you there and back quickly.
• Click the Zoom In Time button to increase the horizontal zoom level to show more event detail.
• Click the Zoom Out Time button to decrease the horizontal zoom level to show more of the timeline.
• Click and drag the area between the buttons to zoom in or out on the timeline.

You can also use the Up/Down Arrow keys to zoom:
• Press the Up/Down Arrow keys to zoom in horizontally in small increments.
• Press Ctrl+Up/Down Arrow to zoom in large increments. If a time selection exists, Ctrl+Up/Down Arrow will zoom to the selection.