Is it safe to take viagra before vigorous exercise eg a 10k run, or are there any potential risks?

Viagra (sildenafil) relaxes muscles and increases blood flow to particular areas of the body. Since there's a risk of increasing your BP with both a drug and physical exertion, I don't recommend it.

Here are the cardiovascular side effects and frequency. Note how uncommon most are:

Very common (10% or more): Flushing (10%)
Uncommon (0.1% to 1%): Heart rate increased, palpitations (heartbeats felt), tachycardia (increased heartbeat), hypertension (high BP), hypotension (low BP)
Rare (less than 0.1%): Myocardial infarction (stroke), atrial fibrillation (part of your heart not in rhythm), sudden cardiac (heart) death, ventricular arrhythmia (heartbeat out of rhythm), unstable angina (chest pain)
Frequency not reported: angina pectoris (chest pain), AV block (blockage within heart), postural hypotension (low BP related to position), myocardial ischemia (heart muscle without enough blood circulating), cerebral thrombosis (clot in brain), cardiac arrest (heart attack), heart failure, cardiomyopathy (heart muscle dysfunction), shock.
Postmarketing reports: Serious cardiovascular, cerebrovascular (brain), and vascular events, including myocardial infarction (stroke); cerebrovascular hemorrhage (stroke); transient ischemic attack (small stroke); hypertension; subarachnoid (in certain areas of your head), intracerebral (in certain areas of your brain), and pulmonary(lung) hemorrhage have been reported in temporal association with the use of this drug.