What an interesting variety of questions you're asking about how to invest your money for the long-term.
A Roth IRA is a great way to save money that will grow a lot, and that when you withdraw from it, anytime after age 59.5 it is all tax free. You can withdraw your initial investment before age 59.5 without any tax penalty.
The Roth can be with Vanguard, Fidelity or a brokerage like TD Ameritrade. You can move your Roth from one to another, from your Fidelity to Vanguard, or to TD Ameritrade, Scottrade, etc.
You can choose to invest in passively managed index funds, or you can get individual stocks, or bonds, or exchange traded funds.
Vanguard and Fidelity both have brokerage account options where you can trade stocks or ETF's, but Vanguard and Fidelity are better known for the no-load mutual funds that they offer you.
You discussed a desire to get stocks paying high dividend yields. So one way a company can reward shareholders is by giving away accumulated cash to shareholders in the form of dividends. Or the company can hold onto the cash, like Apple, and the price of the stock goes up from when you bought it.
If you are going to save money in your Roth IRA there's no advantage to having dividends because any kinds of income from your investments such as dividends or capital gains will not be taxed. Qualified dividends are sought after by retired people or rich people who wish to pay lower tax rates for that source of income. That's why Warren Buffett said he was paying a lower tax rate than his secretary.
Someone mentioned that if you are in a low tax bracket your investments don't have to be in a tax-advantaged retirement account (your Roth, 401k) because you will pay no taxes on capital gains. That's fine for now if your income is lower, but you're 24 years old. You may end up getting a high-paying job at some point, and then you would have to pay capital gains taxes on your investments. Also if your take home pay is $4,000 a month then you are in the 25% tax bracket and you would have to pay taxes on dividend and capital gain income.
So I recommend you continue to invest in your 401k and your Roth IRA. Does your company match some of the money you put into the 401k?
Figuring out which companies are going to outperform, which stocks will give you the best dividends, and when to jump into and out of the market are impossible to figure out. For one thing people with advanced degrees in Finance/Accounting who understand the financial statement jargon of publicly traded companies are still unable to beat the return of the low cost Total Stock Market Index Funds that are offered by both Fidelity and Vanguard. Secondly, a lot of the chief executives of companies have ways of manipulating revenues, expenses, and thereby affecting the stock prices, that it makes relying on the financial statements not a good idea. You could pick the wrong company. Rather, it makes more sense to spread the risk of stock ownership over the thousands of companies represented in a total stock market index fund, whereby your success is based on economic trends.
The disadvantage of splitting your Roth contribution between Fidelity and Vanguard is that if you have a low amount of money invested with them, then they will charge you certain fees. Once you reach a threshold amount the fees are removed. At least that's the way it used to be. If that's no longer the case then by all means put some money with Vanguard, that's what I use.
Finally, in terms of when to go into the stock market, it's hard to time that. You can choose to do something called dollar cost averaging, whereby you will put $500 a month automatically into a Total Stock Market Index Spartan Fund in Fidelity, until you reach the $5500 you can invest for Roth 2014.
And the choice on how to allocate your investments should be based on a model that you have chosen and will stick with. Younger people tend to go with a higher percentage in stocks versus bonds/cash. So you can be in 70% stocks, 30% bonds as one example.
So your money in the Fidelity brokerage trading account, Roth, and 401k is your money for the long-term. The trading account money you can withdraw at any time without tax penalty. If you're planning to retire before 59.5, then that is the money you would presumably use to live off of.
Your asset allocation for this which amounts to about $23,550 to which you plan to add another $2750 to the Roth brings you to $26,300. 70% of that could be in a total stock market index fund, 30% of the rest could be in a total bond market index fund.
The other $16,900 is your emergency fund and can also be the beginning of the savings for your down payment to buy a home. That should remain in cash and not be invested because you don't want to run the risk of losing it in the volatile stock market.