Hi,
There is no such thing like "increasing the zoom rate". Zooming the picture virtually won't give you any nice result. Even Adobe Photoshop can't do it, because we still don't have that technology in the market. When the picture scaled up (virtually), the picture will start being blurry and you will start seeing the pixels on it, which is not a good thing.
As it states here:
www.flexiblewebdesign.com/Virtuemart-Modules/Virtuemart-Zoom-Effect-Plugin
NOTIFICATION: You will need to upload some large product images. If you upload small or medium-sized (less than 500x500 pixel) product images to your shop, the zoom effect won’t be adding much user-exprience to your website. Zoom Effect uses the original images inside the zooming-box window, it does not virtually scale up your original images to avoid picture quality loses.
regards.