You can go for topical application of cream containing aminophylline, isosorbide dinitrate like vasodilators which were used experimentally to treat erectile dysfunction (http://www.ncbi.nlm.nih.gov/pubmed/8646143)

Better alternate I would like to suggest that instead of applying such topical vasodilator on tail just apply the pressure on tail by any means eg. applying a knot of thread, tighten the same, it will cause congestion of blood in tail vein which make easy to collect the blood.

If you have to collect blood only once from the rat & in large quantity then you can also go for Retro-orbital plexus method. 

Good Luck...

I suggest you to use warm water instead of topical vasodilator. Just immerse rat's tail in beaker field with water for like 1 minute and the vasodilation will occur. 

I also use warm water to aid vasodilation when performing tail vein injections - works well

phenoxybenzamine, but warm water is much safer. When desperate, xylene will work but is toxic and not safe to handle in a casual manner.

I also suggest you to use Warm water to aid vasodilation for tail vein injection.

I suggest you to use ethanol to aid vasodilation for injection

 In the old days (seventies and eighties Xylene was used locally  but it is irritant to the animal's skin and may be toxic.In medicine Nitroglycerine at low concentration  is a strong venodilater when administered  under the tongue (Nitrolingual 0.4 mg spray). It may be worth considering as an idea it may help dilating amm veins when administered under the tongue. You may also try Ether locally but you will find that it is volatile and disappears quickly from the site.

I also use warm water to aid the vasodilation which works better while doing tail vein injections

There are many options like 1. Warm water very easy ,economic and effective 2.Rubbing Alcohol is also effective 3. Nitrolingual spray (Glyceryl triNitrate 0.4 mg).

Warm water would be a good idea but slow heating over time results in a "die away" phenomenon which describes the decrease in skin blood flow over time with slow heating - in human skin at least. Maybe worth considering

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2637777/

Warm water is ideal.

I was going to say “heat” as well, but if you need a chemical agent with specific vasodilator effect, try phenoxybenzamine. There are also many irritants that will do the trick, with obvious undesirable side effects. Also, ask a nurse about techniques they use to visualize and engorge small surface veins for injections. They’ll probably have some suggestions, too.