No supplements necessary.
Here’s the conclusion of the study below:
In the absence of convincing evidence for the deleterious effects resulting from the lack of DHA from the diet of vegetarians, it must be concluded that needs for omega-3 fatty acids can be met by dietary ALA.
DHA Status of Vegetarians, Prostaglandins, Leukotrienes, And Essential Fatty Acids, August – September 2009
ALA is alpha-linolenic acid, an omega-3 (n-3) fatty acid found abundantly in plant food. The body uses it as a substrate to make EPA and DHA. Here’s a list I made in a prior post (A Vegan Can Get Enough Omega-3 Without Taking A Supplement) of the total omega-3 content of some common plant-based foods, most of the n-3 is probably in the ALA form. Even very low-fat foods like spinach and cauliflower contain respectable amounts of omega-3.
Walnuts, 7 halves, 1282 mg
Flaxseed, ground, 1 teaspoon, 570 mg
Chia seeds, whole, 1 teaspoon, 350 mg
Kidney Beans, boiled, 252 mg
Frozen spinach, 1/4 cup, 176 mg
Brussels sprouts, 1/2 cup, boiled, 135 mg
Cauliflower, 1/2 cup, boiled, 104 mg
Broccoli frozen, not cooked, 82 mg
Pistachios, raw, 25 nuts, 36 mg
Oatmeal, dry, 1/3 cup, 27 mg
Red sweet pepper, 1/2 cup, 19 mg
Zucchini, 1/2 cup, cooked, 12 mg
Here are a few more bits from that study. On cardiovascular disease:
It has been postulated that an increased intake of DHA may decrease risk of cardiovascular disease, possibly by affecting the susceptibility to cardiac arrhythmia . However, obesity/overweight is less common in vegetarians and vegans than omnivores and blood pressure tends to be lower . Serum total and LDL cholesterol concentrations are also lower among vegetarians and especially vegans [2,8,15] compared with meat eaters, which would retard the development of atherosclerosis. This observation is consistent with the favorable effect of a strict vegetarian diet on atherosclerosis progression .
DHA supplements can increase LDL:
It is notable that Geppert et al.  reported a 6.9% increase in LDL cholesterol with 0.9g DHA/d in vegetarians. We reported a 7.1% and a 10.4% increase in LDL cholesterol with 0.7 and 1.5g DHA/d in omnivores [35,36], respectively.
Giem et al.  were unable to demonstrate any increase in risk of dementia in Seventh Day Adventist vegetarians. Furthermore, we have been unable to find any evidence of cognitive impairment  in vegan men — the National Adult Reading Test , which is a standard test to measure pre-morbid IQ, was on average 119 (range 101 to 128) in 118 vegan men.
Vegans are a healthy lot.