I’m not sure on this one. I think vouchers is better – if people are not buying “a thing” then they tend to want to know how the money they are putting in is going to benefit the couple, and not just get spunked in the pub.
Last couple of weddings I went to the couples asked for cash towards their honeymoon. OH and I didn’t put anything in, simply because it cost us to travel to attend these weddings, stay over in a hotel etc, we couldn’t afford to fork out on top of that. We explained it to the bride and groom and they were fine with it. The last wedding we went to OH was an usher, had been on the expensive overseas stag do, paid for his own suit hire, we’d paid for the travel, hotel room etc, so we just couldn’t afford to spend any more on it. So if you are going to ask for cash, be prepared that some people may not be able to afford to contribute in that way.