How long since you stopped the depo and how long did you use it? How old are you? Are you in a hurry to menstruate? Why?
Herbs are not drugs, they work by assisting the body back into its own balance and processes. So rebalancing after artificial hormones is a process. You can assist this process in lots of ways and I'd suggest looking at building good health before taking herbs with possible hormonal actions (and doubly so if you are wanting to conceive). What's your general health like? What kinds of foods do you eat? Exercise? Stress? etc.
An answer to your original question:
my question is is dong quai strong enough to work on its own without the vitex??
It depends on your body and what it needs. AFAIK dong quai works by actions on circulation and what's known as blood moving. It's also a tonic. i.e. it's not regulating your cycle hormonally. So if what your body needs at the moment is what Traditional Chinese Medicine views as moving blood then dang gui might be a useful herb. But if it doesn't, then dg might not be useful or even counterproductive. Vitex works on the endocrine system and does have some hormonal regulatory effect on the cycle, but it comes with risks. Personally, I think you are experimenting without really knowing why or what you are wanting to achieve. I'd be looking at the specific ways that depo affects the cycle and body, and how the body can come back from that. A herbalist experienced in women's health could help, or you could research this yourself. Let me know if you want some resource references, but it'd be good to know what information you are using already.
It's all good (except for the crappy bits).