None of my clients that were performing online surveys had the resources to do any of the likely approaches for verifying invited respondent was the actual respondent. The main reason in my case they were doing online surveys was the cost was much less than other methods such as telephone surveys.
I am not aware of anyone doing surveys that have video records of survey participants. I won't say there are none, just that I am not aware. The video wouldn't mean anything if you did not already have some video of the intended respondent to compare for identification. Even at that I could see a camera capturing an individual but not the key board of responses.
If you want to do something like that I might suggest contacting the people that do the SAS certification exams where they have some goal of making sure the person getting the certification is the one taking the test. I doubt of the service is inexpensive.
Incentives (your vouchers) may have bias but how much and is the additional information gained by having more respondents than you might have without the incentive. Lots of arguments on both sides. If a target audience is hard to reach incentives may be the only way to get enough sample but only if the respondent thinks the incentive is worth it.
