There are few variations of the vaporizers and most of the components are the same to rebuild them. If you have done the Impco ones then you should not have a problem. Just make sure you have all of the correct components. Nissan offers a complete kit but it will only work on some of the original versions. One of the biggest things is making sure that the pressure settings are correct. I believe that if you can get a copy of the service manual section for the vaporizer it will give you step by step process and the correct pressure settings.
As far as the grade of fuel a lot of the LP suppliers use what they call farm grade which would be similar to what is used with your LP grill and is not very clean so it has a tendancy to build up the sludge or tar. The reason they go with this because it is cheaper and most people are not going to know what is being sold to them unless they ask or know to ask. I am more on the parts side but there should be other techs on here that will chime in and give you other things to check if it still continues to happen.