When shoppin around for different lenders, a general rule of thumb is that your credit score being run by more than 1 within a 30 day period will not have an impact on your credit. Shopping around to find what is best for you is encouraged.
Pre-approval is good to have, but you can get it done in a very short amount of time, and you can do it right before you are ready to start looking at homes. The only thing you might want to consider is getting preapproved sooner than later so if you don't qualify, you will know exactly what needs to be done in order to qualify in the future. There are many things besides credit that are considered, so you want to be sure you have everything in line.
Speaking with a lender over the phone might even get you started on the right path to doing the most you can in order to be approved the 1st time. Preapproval should be no different with the VA certificates, but check with a lender to be sure.
The best thing that you can do is pick an lender who will help you through the process of getting a pre-approval. The lender should be able to direct you to do certain things that will make your credit scores rise as well advise you as to those things which can damage your credit scores. For example, most people who have less than perfect credit think that closing credit card accounts will help them, when actually it does the opposite. Below is a site that I found online which might be helpful to you.