T.M.
Let her sleep where she wants and how she wants/feels comfortable. There is something wrong with her being so upset she's vomitting. That is very troubling to hear. Bottom line, let her ease into things at her own pace and let her get a good night's rest. She needs more support from you and a nicer bedtime routine that she dictates, not you. Put her needs and anxieties first and it will all work out. And continue to do so even after the second baby arrives.
Both my kids went from crib to twin/queen beds. They wanted nothing to do with the beautiful toddler bed they had. My 24 month old daughter felt most comfortable in a queen bed, so we went ahead with it and no guardrails. She likes being independent, using her step stool to get up and down. She does wake up once in awhile looking for comfort from me, husband, or even her brother, but both my kids are like that. My son (now 4 1/2), went to a twin bed at 2 1/2 years old.