As you probably know, I don’t give medical advice and you should not trust medical advice given over the internet, especially by someone who hasn’t seen you and doesn’t know your full history. You have said that your problem is caused by a varicose vein on your inner thigh: what symptoms are you experiencing? Is the vein obvious, blue, spidery, or thick or gnarly? What type of pain is it causing? When does the pain occur: after sitting for a long time in a posture (and which postures?) or right away? Does it only occur in the evening, after a long day of standing or sitting still, or in the morning as well?
You see - there are a lot of possible causes of your problem, and without knowing the cause, it is challenging to suggest a cure. But let’s look at some generalities of varicose veins. A vein is the part of the blood circulatory system that returns blood to the heart and lungs. Since the blood in the veins is depleted of oxygen, it no longer has that nice red look but rather venous blood is blue. There is a long way for the blood in our legs to go to get back to the heart, and the heart is not strong enough by itself to pump the blood back up, so veins employ valves that close behind the blood to prevent it from falling back down the legs. Moving the leg muscles assists in pumping the blood back up to the heart. However, if we are standing for long periods, or sitting stationary for a long time, the blood doesn't flow through the veins and may pool there. This causes stress in the vein walls, which can distend and enlarge the veins. Pregnancy and hormonal imbalances can also cause the veins to swell. Women are more susceptible to varicose veins than men, but men do get them. Aging can do it as well. Often there is no pain associated with varicose veins, but as you have discovered, sometimes there is pain: burning, aching, cramps, itching can all occur. Very rarely a blood clot can form, which is quite dangerous, and this is one reason for having a medical professional check you over if you do have varicose veins.
As we just saw, there are many possible causes of your problem. Depending on the cause, my suggestions to you would differ. But, sticking with generalities, sitting for a long time in one position is not a good idea for varicose veins. You need to move your legs to get the blood flowing again. You may want to modify your Yin Yoga practice to have shorter holds, but repeated holds. For example, do Half-Butterfly to your right side for only 2 minutes, then do the left side, then return to your right side for another 2 minutes, etc. Don’t remain in any one seating pose for more than 2 minutes at a time. Experiment. Maybe you will need to change every 1 minute. Maybe you will need to skip all seating postures! You will have to find out what works for you.
On the other hand, you may want to add some postures that help the blood flow: Snail pose, or Wall Yin
may be ideal for you. Being upside down helps the blood to move. You can probably safely do Reclining Twists, Sphinx/Seal, Dragons and other non-sitting postures as well. You asked specifically for hip openers: I would again avoid seated hip openers (no Shoelace or Butterfly) and try Winged Dragon instead. Maybe Swan will be okay but you’ll have to play with it.
Again, take such advice with several grains of salt: test the ideas out carefully paying full attention to how they feel both while in the posture and in the next 24 hours or so. But, do let us know how it goes. I am sure others would be interested to hear how you do.