VantivCallout (@future) Code:

public class VantivCallout {

    @future(Callout=true)
    public static void VantivPDCallout(Set<Id> pdOppIds) {

        List<Opportunity> oppsList = [SELECT Id FROM Opportunity WHERE Id IN :pdOppIds];

        HttpRequest req = new HttpRequest();
        req.setEndpoint('https://url.some-site.com:PORT/Lead/Submission');
        req.setMethod('POST');

        String username = 'NOT-A-REAL-USERNAME';
        String password = 'NotOurPassword123';

        Blob headerValue = Blob.valueOf(username + ':' + password);
        String authorizationHeader = 'BASIC ' + EncodingUtil.base64Encode(headervalue);

        req.setHeader('Authorization', authorizationHeader);
        req.setHeader('Content-Type', 'application/json');
        req.setHeader('Accept', 'application/json');

        Http http = new Http();

        JSONGenerator generator = JSON.createGenerator(true);

        for(Opportunity opp : oppsList){

            generator.writeStartObject();
            generator.writeStringField('OwnerFirstName','Morgan');
            generator.writeStringField('OwnerLastName','Marchese');
            generator.writeStringField('OwnerEmail','mmarchese@millenniumsi.com');
            generator.writeStringField('DBAName','Morgan's Test Account');
            generator.writeStringField('DBAPhone','555-555-5555');
            generator.writeEndObject();

            String jsonString = generator.getAsString();

            req.setBody(jsonString);
            HTTPResponse res = http.send(req);
        }
    }
}