There is a difference with bith options for fraternal and ID twins.
Fraternal twins are considered "safter" to deliver vaginally because there are 2 palcentas (althought they can merge, no BLOOD vessels can merge), so it is really like 2 separate singelton births, one after the other.
The risk in ID births is do to with the SHARED placenta, thus the sharing of blood vessels and blood supply. Twin To Twin Transfusion Syndrome (TTTS) is a rare, but extemely dangerous disorder of the placenta in identical multiples. It happens when there is uneven blood distribution between the twins. One twin becomes a donor and the other a recipient. 10-15 years ago, this diease was not detectable let alone treatable and many twins died inutero or during vaginal births. Detection is via ultrasound of linked blood supplies. If detected before 16 weeks, nothing can be done and most twins die. Between 16-25 weeks, laser surgery can be performed, but this is dangerous and only some pregnancies survive. The uneven blood vessels are lased away to make the flow even. If TTTS is detected after 25 weeks, delivery is the only way for the twins to survive.
Then there is the Acute form of TTTS. This form can happen literally within hours. The blood flow reverses rapidly and both twins can die within hours. One day you may have a routine scan with no detection and 24 hours later the babies can be dead. Luckily I had a scan at the right time when acute TTTS was detected and I had my girls 20 minutes later. My recipent twin had a liver 3 times normal size and had begun organ failure. I am blessed both girls survived.
The danger of vaginal births with ID twins is that the acute TTTS can actually happen during the delivery. The babies are not safe until the second twin is clamped. Because they share the same blood supply, twin one may be being born and the blood supply suddenly reverses and twin two will pass away before being born. It can happen and does happen that quickly. There is a TTTS support group with many very sad stories like this. TTTS is not given the coverage it should, many GPs are unaware if it and every pregant woman should try to have a scan at around 11 weeks to measure the membrane to see if they are ID twins. If they have, they should be monitored by a TTTS specialist and have weekly scans.
I hope that explains why doctors are cautious about vaginal births with ID twins...TTTS doesn't happen with every ID birth, but if you are unlucky enough to have experienced it, you go the safest option.