Their source code for serial communication should be easily portable to CVI if written in C language: they have maybe published a document on their communications protocol that can help you in understanding this code so that you can manage to make it functional in CVI. We have used several instruments connected via RS232 and never have had problems in integrating them in CVI applications. Wether to use VISA or standard RS232 library depends on how you are familiar with each one of them. This is basically the first step in your development.
Once you have ported your code to CVI, you can decide to encapsulate it in an instrument driver, wether to do it or not depends on how much you will be using this instrument: in my opinion it is not necessary if you are planning to use it in this only application, while it can save a lot of your time in developing if you plan to use this instrument in several applications of yours. Developing an instrument driver needs a good concept design in defining the functions so that they are flexible (i.e. they can used in several application environments) and contemporarily are robust and efficient; in this approach, you can expect time savings only if you re-use such a driver several time so that you split driver development time on all projects.
