Please and thank you go a long way, i've found.
Most of the CNA's I work with don't mind getting vitals, in fact I do think it helps them feel part of the team b/c they're being trusted to recognize and report if something isn't wnl. This becomes particularly valuable in LTC, where there are 40 residents to one nurse on my shift.
I, too was a little nervous about delegating at first. I'll just tell you what I did to help. At first, I just tried to get to know the CNA's a little bit, asked questions about how things usually were run on the floor, etc. to build some rapport and respect. Then I would try to ask for stuff in a direct way, but with some leeway, such as "would you please get these vitals when you have a minute?" so they know you understand they're busy, too.
Oh, yeah, the more you do for them, the more they'll be willing to do for you. Just little things like filling a pitcher or toileting someone can make a difference. I'm sure you do these things anyway.
It won't come all at once. Building respect and trust takes time. But soon you'll find that blend of respect and authority that will serve you well when things need done.
Me, I'm still learning. Most of the time, I'm comfortable with delegating and supervising, but there's one particular older CNA at my job that's so wise and experienced, I still get nervous when I have to ask her for something!