Hi, and welcome to the forums.
Two useful UK sites on financial issues are
http://www.moneysavingexpert.com/http://monevator.com/Which? also does financial info but you need to pay a subscription (there should be a cheap introductory deal which you could then cancel).
I'm in no way a UK investing expert. But -
The UK equivalent of the IRS (I think you meant IRA) is an ISA. You put after-tax money into this, but capital gains and income are free. ISAs are either cash ISAs or stocks and shares ISAs. You can put about £14k into one this tax year and £20k into one next tax year. You going through an ISA provider, who will be a bank, building society or other financial institution - check their charges and the funds they offer before signing up - the sites above will do comparisons for you. You will be able to find Vanguard funds being offered by various ISA providers.
The equivalent of a 401k for you as an independent contractor is probably a SIPP (self-invested pension). Much the same applies: you need a provider and to make decisions on investments. The money that goes into this is income tax free, the money that comes out is subject to income tax. The advantages of this are that you get the tax-free investment increases on tax free money while it is invested, and you may be taking pension out when your income is lower and therefore your income tax rates are lower.
You are right that if you invest directly with Vanguard in the UK you need a minimum of £100k. That is probably not for you, unless you are already maxing out your tax-advantaged options through a stocks and shares (ie you could put £34k of your £100k into an ISA within the next few weeks, and a lot or all of the rest into a SIPP - although you would need to check that amount against your earnings to see how much of it could be income tax free this tax year and next).
Investing in stocks and shares is a "buy and hold" operation: if you are expecting to need the money in the next year or two, for instance to get back into the housing market, you should find the best cash investment you can. Remember that the UK government guarantees only the first £85k of your savings in any one institution, so it might pay you to split it between two different institutions - although a lump sum of £100k might get you a higher interest rate.