I have major problems with chronic constipation myself and I am currently pregnant myself...and i'm also mostly a vata type. Being pregnant limits option in what you can take safely. I love triphalia for constipation but it is definitely not recommended for pregnancy as you said, bummer!

what i've been taking recently is one tbl of freshly ground golden flax seed and a couple pinches of slippery elm bark in about 10 ounces of water daily. Put the slippery elm in the water first and let it sit for 5 minutes, then add the ground flax seed, shake up and drink down. (You can also find slippery elm in capsules as well which you could just take seperately). It doesn't taste bad and it works very well. I go first thing in the morning if I do that the day before. I'm fairly sure the flax seed is ok for pregnancy in moderation, in fact it has the good omega 3 fatty acids that are good for babies brain development. I find the freshly ground flax seed is much more effective than the oil and it tastes a world better, i can't stomach the oil. The slippery elm bark is a question mark though I havent seen any thing that says that its unsafe for pregnancy.

Also there is a great cookbook called the ayuvedic cookbook you can find on amazon. I might suggest you limit your raw veggies, while raw veggies are great for pittas, they are very agravating to a vata's weak digestion. vata's tend to have low agni or digestive fire so steamed veggies would be much better and easier to digest. also i find the magnesium products to be agravating to my digestion. a little magnesium is great, but a lot used as a form of laxative i find to make things worse for me. they do "clean" me out but then they disrupt my digestion for days afterward.