Are there any possible concerns for vegan vegetarians who consume ALA from plant sources without any DHA/EPA in their diet?

Very few studies have been conducted to address your important question. In general, lactating vegan women have significantly lower levels of DHA in their expressed breast milk as compared to non-lactating vegans. Many years ago, Dr. Conquer and myself conducted research in groups of vegetarians and showed that supplementation with an algal source of DHA could significantly improve circulating blood levels of DHA and increase DHA levels in breast milk. The complete reference for some of this research is as follows: Conquer, J. and B. Holub. Docosahexaenoic acid (omega-3) and vegetarian nutrition. Vegetarian Nutrition: An International Journal 1(2): 42-49, 1997.