As citadelgrad mentioned, there are currently agencies that certify vegan standards. Vegan Action's certification (the V in the heart) is no longer accepting new applications for certification. According to this article from Vegetarian Journal, other certification groups include the European Vegetarian Union (not vegan), Natural Food Certifiers, The Vegan Society, and a symbol from Edward & Sons Trading Company.
To discuss the broader question, as I understand it, the issue with food certifications is twofold.
First, companies need to feel it is worthwhile to pay to get this certification. There needs to be enough of an advantage to sales in getting the certification that it outweighs the cost in both money and time to get it. (Certification agencies get paid for their symbols.) Which means that there has to be a large audience who will be significantly more likely to buy a product if it has the certification than if it doesn't.
Secondly, there needs to be an agency to provide the certification. This requires knowing your audience well enough to establish standards that are widely acceptable. Then you need to train inspectors to monitor the food production to ensure that products are living up to the standards you set. And they need to publicize the value of the certification among the desired audiences. (Often different groups. For example, kosher certifications are often promoted as providing more healthful food -- which may or may not be the case.)
The process is complex. For example, there's currently a movement in the kosher community to found a certification of ethical production. (Wikipedia discusses it here.) This process has wide support among many kosher food consumers, and yet it's still in development and has been since 2006.
It appears to me that there has not yet been a symbol that meets both criteria well enough to become common for vegan foods.