It depends upon what you mean by "early decision." There are several vet schools that offer "guaranteed admissions programs" for talented high school seniors. These schools include Kansas State University, Mississippi State University, the University of Missouri-Columbia and Purdue University. Admission to these programs is very competitive. To qualify for application to such programs, typically there is a high school gpa/class rank and/or ACT/SAT cutoff below which you are not eligible to apply (for Purdue's program, for example, you have to be in the top 10% of your high school class AND have an ACT composite of 28 or higher or an SAT of 1950 or higher). If you meet the cutoff(s), you complete and submit an application to the vet school, including written references (usually two academic references and one from a veterinarian), a summary of your animal related work experience, high school transcripts, etc. Competitive applicants are offered interviews and the most qualified applicants are offered admission to the program. In other words, it's basically the same as the normal admissions process to vet school without VMCAS. Now, the catch(es):
1. First, these programs usually require that you obtain a bachelor's degree in a specified major (usually animal science, biology or biochemistry) from the university with which the vet school is affiliated in order to be eligible for a seat in the program. For example, in order to be eligible for a seat in the guaranteed admissions program at Kansas State University's College of Veterinary Medicine, you have to attend Kansas State as an undergraduate and major in animal science, biology or biochemistry. Note that with many of these programs, you won't know whether you are accepted to the program until you are already in your first semester as an undergraduate at the university. As such, you must be prepared to attend the undergraduate program at the university with which the vet school is affiliated even if you are rejected from the early admission program.
2. Second, you still have to complete all of the prerequisite undergraduate courses for the vet school, usually with a minimum grade of C in each course.
3. Third, you have to maintain a minimum cumulative undergraduate GPA (usually somewhere between 3.3 and 3.5).
4. Fourth, you still have to take the GRE and usually obtain a minimum score.
If you don't complete these conditions successfully, you're dropped from the program and you lose your guaranteed seat, although you can still apply to the vet school through the regular admissions process. I know someone who got into the early admit program at Kansas State. To give you some idea of what they are looking for, this kid went to a magnet agricultural high school and had the following qualifications: completed four years of animal science courses while in the high school program, plus all AP science courses; 4.0 high school gpa; 31 ACT composite (98th percentile); completed her first year of college while in high school (combination of AP credits and college courses taken while in high school); vice president of high school FFA chapter in sophomore and senior years; won 10+ FFA competitions, including several gold medals at the state and national level; 10,000 hours animal related work experience with everything from lab animals to horses to exotics, documented from grade school (no, I'm not kidding); worked for a vet her entire senior year of high school; started her own successful equestrian training business while still in high school; first chair oboe player in her high school orchestra; won two local merit scholarships for college as a high school senior, and then awarded two more merit scholarships from K-State for undergrad; tons of community activities including Habitat for Humanity, church youth group, Girl Scouts, 4-H, etc.
There are also some other vet schools with programs that guarantee admission (with similar conditions) to college freshman and sophomores. As best I recall, these are offered by Cornell University, the University of Minnesota-Twin Cities and Tufts. There may be others as well. Your best bet is to look at the admissions section of the websites for the vet schools that interest you. Good luck.
